never towed 2 at once, but i can tell you from my experience of towing vehicles, a few things. Trailers are rated at weight levels. My particular tralier is a low deck 7K pound car hauler. trailer itself is 1200lbs. so i have a useful load on the trailer of 5800lbs. now my jeep is heavily moded, so i figure (never weighed it) it is between 4000 & 4500 lbs. so when loaded on my trailer it has a little bit of useful load left for gear and supplies.
What you have to take into consideration is GVW of the trailer that you want tow and the weight that you are going to put on it compared to the empty weight of th trailer.
For instance i have rented a few trailes before i bought mine. Once i rented a 20' flat bed with an 8' wide deck to tow jeep with (only thing trailer place had) that trailer had a gvw of 10K but it was very heavily built. the guys at the trailer place said that it was about 3000lbs empty, and i belived them once i pulled it you could tell it was there, unlike my little 1200 lbs car hauler.
so getting to the point here,
even though i'm sure that i probably could have fit two jeeps on that 20' flat bed i'm positive that it would have over loaded it, had the jeeps been the same weight as mine. and when trailers are over loaded things just aren't safe. braking becomes a problem and tires are over loaded its just something to be avoided at all costs. also want to make sure that you don't exceed the GVW of your truck. my trucks GVW is 16K truck and trailer, so my truck is 5600 lbs with a full tank of gas and the wife in the passenger seat, add trailer 1200 Lbs and jeep 4500lbs and 500 lbs of supplies and gear and i'm at 11800 lbs. well below my trucks limit, but if i were to add another jeep and a heavier trailer than i might have a problem.
Just my .02 and experience
Jerms