What 5th wheels have strong frames for double towing?

Paul, I think the biggest issue you may face is the tongue weight of the rear trailer unloading the fifth wheel pin to the point of making the entire combo unstable. Having the brake controller capacity to operate all those brakes could be an issue as well. Reluyog's tow vehicle is designed to tow about 8 times as much weight as yours so he likely doesn't have an instability issue.