I double tow quite a bit. I'll include a couple of pics of my rigs. It's usually more stable to have a 5th wheel plus bumper tow setup (first picture below), since the weight distribution of the 5th wheel puts the center of gravity much further forward of the wheels. If you're going to double-tow with 2 "bumper pulls" in tandem, you will probably need a sway control somewhere in the mix. I used a sway controller between the boat and camper (second picture) in order to make the trailer pair react like a rigid (but long) single trailer.
No matter what people tell you, the stability of the trailer is related to the position of it's CG vs the axles. Moving the CG forward increases stability, and positioning it right over the axles (as in most non 5th wheel setups) makes it less stable. Sway controllers are cheap insurance. If you're double towing with conventional hitches, you'll really be better off with the sway controller.