What 5th wheels have strong frames for double towing?

I don't expect the manufacture to say the limit that the trailer is rated to tow because of liability reasons.

I talked to Northwood last week about their Arctic Fox and was pretty pleased after I talked to them. They said you can add a hitch to the rear of the trailer as an option but the reason they don't rate it is because you can't double tow in Oregon. It is constructed with a 12" I-beam frame and looks very stout.

There is still some research to do but so far the Arctic fox is at the top of my list.