Sounds pretty close to what I'm thinking about doing as well, in the same vehicle, with 2 SSA XCon 12's
That's going to require a whole lot of room though, and I'm not sure I can give up that kind of space.
I'm looking at a 6th order, 5 cu/f @ 28 hz + 2 cu/f @ 48 hz. Every 4th order I model in winisd looks way too peaky. Although, that's modeling the xcons, not the mofos, so your mileage may vary.
Good luck, I'm looking forward to seeing a build log!