A friend of mine offered to let me ride her horse while mine recovers from a injury. We are both riding at first level. She rides 4 times a week and I would be riding 4 times a week, so there would be some days we are both riding. One in morning and one in evening. The horse will have at least one day a week off. I know some of the top trainers say this is perfectly okay for the horse, as long as both rides are short - around 30 minutes or so. Has anyone done this and had a good or bad experience?
School horses do it all the time, often for two hours a day. Just remember you're asking the horse to be very tolerant of possibly different styles and different aids, don't expect the sharpness you might for a show-ready tuned-up (even 1st level) horse.
No reason not to do it, horse gets more fit, and learns by having frequent reinforcemnt. Usually rode 2x day 7x a week. Ideally one day is hacking, one jumping or caveletti or etc. Sounds doable if you are both at the same level.
I did this with my first dressage horse. As long as you both ride consistently the same, the horse will stay very fit. The problem that I ran into was the other rider rode a little tilted, so the horse got in the habit of riding a little on the forehand. He liked doing that.
I suggest going out and getting a planner or notebook so you and your friend can easily keep track of what the other did that day, so you are careful not to overwork the poor guy, especially at first. You can keep it in your trunk or grooming box. And it will be a nice way for you to take notes on your ride! I had 2 students who shared a horse for a while, and it worked out very well. The horse gains weight off of the smell of food, so it was a good way to get him fit.
I work my mare 9 times per week. She needs it to keep from getting the crazies. So I don't think 8 times is too mcuh. But only 2-3 of those sessions are really strengthening sessions. I think if you keep the focus on the horse and stay very aware of how he's feeling, it will be fine. But keep in mind that if it's hot one day and he got ridden in the morning, he may not have the energy to go out again, and you might have to be flexible and scratch your ride for the day. And your friend needs to be willing to do the same.