Looks like I am one of the exceptions here. I do use a snowmobile trailer for pulling my quads. Mostly out of necessity though because it is simply impractical to own more trailers than I already have.(3) The trailer I use is an 8x12 aluminum (FLOE brand) Being from Minnesota you know about road salt and what it does to steel. After the second time sandblasting and painting my steel trailer I said NO MORE!
Contrary to what others have said I personally have not encountered any bearing problems. I pack them once a year and thats it.
I use my trailer a lot. I pull the trailer weekly to my cabin with either 2 and sometime 3 quads and /or snowmobiles. I have also pulled it to Yellowstone multiple times at extended high speeds (85 to 90 mph)without problems.
Trailers, like any other piece of machinery require maintainance. If you grease your bearings, inspect and inflate your tires properly (and often) and properly distibute your load a snowmobile trailer can be very convienient to use.
As far as loading 2 quads on a trailer alone it is simple. I do it all the time. Drive one quad on to the trailer as far as you can without tipping it. Set the brake, then hook up a tie down strap.
Now drive the second quad on slowly until the bed tips down. Again set the brake, then install the tilt pin. Make sure the load is properly distributed and finish tying down the quads. After doing it a few times with your particular quad or quads you will learn exactly where to place them.
Either way you decide to go, dont rule out snowmobile trailers. They are versatile, convienient and practical (especially if you snowmobile also)
Good Luck