What 5th wheels have strong frames for double towing?

I can't say much about what you are trying to do but contact Northwood and ask them. I do know that there aren't many fivers with a better frame than Arctic Fox. Northwood makes their own frames: no Lipperts in the bunch. They are great to work with and will be upfront and honest with their answers.

When you look at maximum tow weight for your truck--you are going to take a mighty big truck to tow the weight of the trailer and the additional tow.