Thanks for all the replies, guys. In most cases, you've confirmed what I was thinking. I pretty much coming to the conclusion that the two best answers, if I want to take the larger tractor, are to either:
A. Buy a used brush cutter and leave it at the property, hauling the tractor when needed, or
B. Make two trips at the start and end of the planting season; one trip to take/bring back the cutter and one trip to take/bring back the tractor.
Either of these options is cheaper than buying another trailer and hitch and will allow the shell to be left on the truck.
I will have to look at the 20' trailer again, but I had already thought about letting the cutter hang off the back. However, the trailer has ramps and I don't think they are removable. I think I would have to make longer bars to keep the ramps up when traveling and probably remove the cutter tail wheels. Even then, it's questionable whether the cutter would fit without hitting the ramps.
Again, thanks for the help.