Bolder horse should be in front. He has to be brave to face anything you meet by himself. Learn to use your whip with skill.
Lash should be long enough to reach the saddle of the Lead horse. I suggest getting a LONG stick for the whip, seems to be easier to use skillfully with such a long lash. YOU need to be able to send lash out, TOUCH the Leader and bring lash back to coil up on the stick, NOT tangle on the Wheeler or wheels of the vehicle. This can take a bunch of practice, getting lash and tip to go exactly where you want it going if needed for the Leader or a stray dog. Don't want to hit the Wheeler by accident, if he didn't do anything wrong. Wheeler NEEDS to be comfortable and QUIET as the lash goes by him.
Get in a lot of practice with a reinboard, probably while doing whip practice hitting targets. In Tandems a situation will go bad in a HEARTBEAT!! You are going along just fine, then suddenly the Leader is not in front any more! You have NO TIME to look at reins, your hands have to be automatic in responding. If you look down, the situation is probably going to be beyond saving yourself from happening. So MUCH practicing with reins, taking up, letting out, rolling wrists, turning ONLY the Leader for corners, THEN the Wheeler. You are driving two horses, and they need to be driven independently to keep your vehicle out of trouble. You will have all reins in one hand, then do your tweaking, adjustments with the other hand. Reins held in two hands to drive, will be hard on the horses, causing oversteering, not allow correct use of the whip as you need it. When you use the whip, any rein in that same hand WILL be signalling things you don't want to the horses, you drop him on that side. Horse loses the support of balanced feel from both reins. Things could then get lots more INTERESTING than you ever thought possible!
Get proper Leader traces, with straps under belly to prevent them belling out wide to catch a leg or post. Get enough length between Leader and Wheeler, that Wheeler is not running up on Leader.
For a 'correct' look in Traditional Turnout, the larger animal is the Wheeler. However I would still choose the bolder, hopefully faster, horse for Leader job. You only have reins, voice, whip, to control him WAAYYY out front, he has to be confident to move FORWARD alone. You will have a real mess of a tangle, if your Leader won't move out FORWARD when asked.
After you are good with whip, reins, in practice, it is time to practice with real animals. Hope they are quiet and responsive to voice, reins. Do your first hitchings as a Tandem in a paddock with a fence, with at least two helpers on the ground to grab horses if needed. DO NOT buckle your Leader reins together, in case you need to just "let him go" to save yourself and the Wheeler. This stays true even when you get more skilled, reins not buckled together for the Lead horse.
I would suggest using some light, BREAKABLE string for Leader traces on your first several hitchings. Lead horse will be looking for his stopping points by feel, which string traces give him. Helps keep him in front of the Wheeler, feeling the "boundary" of strings. Still easy to break if he gets confused, tries to drop back with Wheeler or comes around to see you! You pat his nose, helpers put him back ahead, retie the string traces for another go. YOU as Driver, need to keep him from pulling traces tight, use your reins well, since Lead horse is merely ornamental! Not really supposed to work or pull, because it will throw the Wheeler off his balance or feet.
As mentioned, Tandem driving is exciting, things happen VERY fast so you have to be thinking ahead of your Leader. Not all horses are Leaders, takes experience in being driven many places, certain personality, that bold going forward since being ahead, alone to face what you meet, is NOT how most horses travel.
Husband says that Tandem Driving is "Heart Healthy" since the blood is rushing thru veins and arteries so FAST it scrubs them clean with your excitement!
My endurance horse was a tandem leader in his former life. According to his prior owner he spent more time stopping her heart (and that of her groom/hubby) than keeping it going!
To the OP: best bet is simply to have every piece of harness, tack, equipment, horses and drivers so intensely sparkling clean and neat you could see yourself reflected in any one of the surfaces. Have the vehicle fit the wheel horse, the harness color and style match on both, present yourself in elegant clean attire, and smile.
I would suggest using some light, BREAKABLE string for Leader traces on your first several hitchings.
All turnout discussion aside, what Goodhors said here is something I definitely ditto ditto ditto!!!!
When I first started driving my tandem, granted they were only minis, but still... having some bailing twine tying the leader to the wheeler was a massive help. And believe me, he broke his trace connection several times. It took a good 6-8 drives before they both got the hang of it.
Definitely had the bolder horse in the lead. With my minis, one is a lazy lump that doesn't want to move, and one is a forward, fun, and super little engine that goes. Needless to say, my pair was not the ideal combination, because you *do* want horses that actually (attempt) to move the same. Had I reversed the two, my bold little leader would have been running on top of lazy lump.
You said yours move the same, but I am sure one is more confident than the other. Definitely put that one first.
But to answer your actual original question, I suppose it depends on what type of show you are doing?
I saw a picture floating around on FB of a pair of ponies turned out in tandem and they looked amazing. Wish I could find again.
Thanks for all the tips and suggetions. They have been driving as a tandem for a few weeks now and they are doing great!
I have one more question- What to do with the leaders harness- Should we take off the shaft holder and overgirth or would it be okay to run the traces thru the shaft holder
I don't know officially, but I would assume for show, the shaft loops should be removed. For practice and at home play, I always left them on, and just ran the leader's tugs through them.
FWIW, I also used a trace carrier through the leader's breeching - this helped soooooooo much in keeping the traces up off the ground, especially on turns, thus preventing my wheeler from stepping over them with her front legs. Of course, this was a mini tandem, so I'm not really sure how it applies to larger horses.
If you are doing a Traditional Turnout, you take off the shaft loops and put on some elongated, flat loops to carry the traces. These flat loops will have a buckle on the top to attach to the strap that holds the shaft loops. There is a longer strap coming off the bottom of the flat loops, that buckles into the overgirth, so these are held snug to the saddle.
In a "proper" Leader's harness saddle, there is a sewn in strap from under the terret rings, makes a long, flat slot on side of the saddle to run traces thru, then is sewn down to the saddle. I don't believe there is an overgirth on that design saddle. Slot keeps the traces snug to the side of the Leader. This design of harness saddle can also be used on a Leader in a Four or Unicorn, I believe. It is kind of old-fashioned, though VERY correct in style on a Tandem. Less folks using it, because it is not adaptable for driving with other kinds of hitches. Easier to buy the pair of flat loops, and of course cheaper, then just modify your single horse harness for Leader use.
Trace carriers on the Leader are suggested ALWAYS. Just know they WON'T keep the Leader traces off the ground if you are using the correct length traces, and snapping them onto the Wheeler short tugs with a Tandem key. Very sharp turns make the traces drop right down, so it is easy to get a leg over a trace.
There is a photo of a Four wearing saddles with the strap slots on the Mischka Calendar for April, at the bottom of the page. All their hames have short tugs, so they don't buckle into the saddle like long tugs do on Hames or Multiples breastcollars. Sorry, you have to have the Calendar to see the little photo, since only the big photos are shown on his site.