One of my brothers used my tractor to stack bales two high before and I did it today. The trailer is a deck over goose neck and a 3R is the smallest that you would want to use. I had no problem lifting 4x5 bales to the top of the loader travel. There is a lot of leverage with a bale that high in the air and the mushy loader control in a 3R cab doesn't help with smoothness, either move the joystick real slow or get some jerky moves that rock the tractor.
The bigger problem is a small cab, with the loader all the way up you can't see it without leaning forward. You are also not as high off the ground as in a larger tractor so it's hard to judge where the bale is. I think I only had 5" or so of loader travel left when setting a bale on top. If you have a larger tractor when loading a taller trailer use it, if not use lots of caution.
A 320R loader has a maximum lift height of 103.3" or 8 1/2 feet, no way you are using a 2 series to load a deck over trailer with two high bales. Also the loader moves in an arc so the higher you go the closer to the tractor the load is. I wouldn't have tried it if I didn't have a cab, way too much weight too high and close to me.
One of my brothers weighed a load of 1st cutting 4x5 hay bales and each bale was about 880lbs. One brother has a 5520 and another a 6 series that the normally use for round bales, also a 95hp JD skid steer, so I will stick to one layer of round bales with my 3039R.