Are you doing 14 x 14? 14x 20 would be better. No matter how friendly they are, one will be dominant and will be trying to scoot away from the other one. Two horses need room to maneuver, and the doorway needs to accommodate two horses going through at the same time. 8-10'. Also if you are walking in there with them, you need to think about your space so that you don't get knocked about if one starts to rush, or kicks out. Two entrances on the ends allow one to come in and the other to leave so that there is no bottleneck. I've had 3 horses in a 30 x 15 turn out and that is tight, but with three doors was workable. 14x14 makes a nice box stall for a single horse.
Good luck
A 14 x 20 would be great, but I don't have enough treated 2x8's for her to go that big. She can't afford to pay the price for new 2x8 boards to go any bigger. I had enough for her to do 14x14 with just a few left over for me in case I might need a few at my home for some repairs down the road. Hopefully there won't be any problems...both of her horses are on the smaller side and get along great...At my farm, my own horses will go into one stall when they don't have to....I've had 4 of them go into a 12x12 stall and just stand there all together, they do it all the time when they have their own stalls...but I do get what you are saying...not all horses get along as well as ours seem to.