I'm in the same situation -- an older trailer usually used for hauling just one horse. I removed the divider, but keep the horse tied. It works well -- the horse has room to choose the angle at which they are most comfortable, plus are able to spread their legs far enough to easily balance themselves.
Two things to consider - is it a ramp or door, and will you be loading and unloading by yourself? The issue with the ramp if you are hauling alone is the loading and getting it closed. If you take out the divider, you lose the butt bars, and have no way to keep the horse in there while raising the ramp. I suppose that tying them would help somewhat.
Right now my 2-horse has no divider, and it made me a bit nervous hauling the trial horse home all of 5 miles without it, but she did fine. I did tie her, but she still managed to get turned around (although she could not straighten her neck - good thing it was a short ride). Her owner said she has always hauled loose in a stock trailer, it was me that wanted to tie her head.
I did manage to unload with no issues thank heavens, because I was alone.
There are friends and faces that may be forgotten, but there are horses that never will be. - Andy Adams
Thanks for the replies!
It has doors, which I would need to change a bit to take out the divider completely. The doors latch onto a center pole.
I would be loading by myself, but my husband will be making a custom buttbar that would just go the whole width of the trailer. That way I would still have a buttbar. It has chains now, so I want to change that anyways.