What 5th wheels have strong frames for double towing?

An area of concern to me would be one I haven't seen mentioned on here and that's the pin box area of your fiver. Adding the leverage stress of pulling and stopping the combined weight of two trailers with 6k additional yanking on your pinbox is not something to ignore.

Consider carefully how that additional load from any angles it might be expected to impart to the rear of your fiver and what that translates to way up front on the pinbox, a notoriously problematic area if not thick stock and welded correctly.