I've seen some that were entire frames the size of the wagon box with a cylinder.
I've also seen the ones that are just barely bigger than the cylinder, with some steel that attached to the front bolester and front stringer of the box. I suspect this is the type you are talking about.
They should handle pressures up to 2500lb hydraulics, so should stand most new tractors.
Typically they bend the running gear middle member before anything else. They are hard on a weak running gear.
You could Tee 2 of them together, but this would work opposite of what it appears you are saying, it would run slower but lift twice as much, and be 4 times as likely to damage something. Also most want to be centered on that middle pipe of the running gear, they wouldn't fit right side by side unless you fabricate more. And, if you loaded your wagon off-center the light side would go up more, causing stability and twisting issues - can be compensated for but the hydraulic connection becomes much more complicated. By teeing them together you still would have full hyd pressure to both hoists, the oil would just be 1/2 as fast.
So - you can double them, but I don't think you'd ever want to unless you had a very strong running gear and box with extra fabricating?