It's actually legal in most states to double pull, with most exceptions being the east and west coast states. Of those that allow it, only a few specify the necessity for the first to be a fifth wheel. I've doubled before, years back, as did my father. Now it's not something you do lightly, and requires patience, lots of it. You can't back up more than 5', in optimum conditions. With my dads rig, he didn't even have brakes on either trailer, back in the 70's and 80's. Obviously we took our time and max pull was decided to be 55 mph, of course that was the speed limit back then. His rig was admittedly lighter and shorter, but was stable as a rock, with brakes being the weak point.
If the weight and balance works out, it will primarily be for 1-6 hour distances. I don't pull a single trailer much past 60mph, so I'd probably pull doubles at around 55. If unstable, I'll have to buy a second tow vehicle, or pack the forward trailer nose heavy. For local, it's probably easier to pull the loads separately. Unlike my dads boat, I can't lift the tongue and pull the boat to the travel trailers rear hitch. That too is where my weight and balance issue may come from. I'll also have to back the TT under the boat which may get the best of me. Wireless camera would help here.
On the TT, my forward fresh water tank adds 350 lbs to the front of the trailers coach. The waste tanks are at the rear and when full may contain as much as 560 lbs. So far, I have towed empty, and loaded with fresh water with empty waste tanks. I can feel the difference in how it pulls. For grins, I filled both waste tanks and emptied the front fresh water. It was still stable although there was more wander when passed by trucks than the earlier test. Only then did I discover I am missing a joint to be able to drain the waste tanks! Doh, waiting on Amazon delivery, so I can drain them.
What I don't know, "yet" is what the tongue of the boat weighs nor the actual boat/trailer weight. So once I have those numbers then I'll know if it's even worth mounting a hitch on the TT.
Fortunately the TT has the axles pretty far back, so the momentum arm is less in the rear than the front, so a full fresh water tank and empty waste tanks may offset the tonge of the second trailer, especially with 60 lbs of propane on the tonge of the TT.
I'm a pilot, and once I have all the actual weights and arm lengths from center of balance, I should be able to determine if my plan will work out on paper to give me 12-15% forward weight on the TT tonge with the boat attached. If I can maintain that and not exceed the TT's gross, then I should be good to go.