Hi,
As long as you are going to all that trouble to fabricate something, why not make it really strong? Weld or otherwise attach a 2" receiver to each of your trailer frames, and then build something to go between them. It can slide out and in as needed. You could even use one of them as a bike rack carrier and the other for something like cargo carrier, or have a wider rack that can carry both bikes and cargo.
With all this talk about firewood, you might want that in the truck and bikes on the back?
Also use care in the weight distribution. If you add to much weight behind the axle, you might take away weight to much from the hitch. Anyway a hundred pounds of bikes will not matter to much.
My pickup truck had a extension hitch for when the camper was on board. It was fabricated by using 1/4" plate steel, with 2.5" square tubing welded to it, and bolted to the truck chassis. 2" square tubing slide into the 2.5" tubing on the truck, and the sliding bumper bolted onto both of these 2" square tubes. I drilled holes for the "normal" and 18" out positions.
If you have something like 2" X 6" tall tubing for a trailer frame, then you can bolt something on to it. However as you tighten the bolts, it will crush the tubing, thus never stay tight. So by using 2.5" long 1/2" steel pipe, drill a 7/8" hole in one side of the 2X6" tubing, then insert the pipe to bolt down the hitch, once tight, weld the insert into place. They will prevent any pressure across the 2X6" tubing while you can still apply 120 foot pounds of torque to the 1/2" diameter grade 8 bolts.
Fred.