Y'all want to home build something big enough to carry two vehicles and ~12k lbs safely at 75 mph for several hours at a time on the highway surrounded by other drivers where it could potentially fail...

I don't see anything wrong with this plan at all... I am not doubting the Ingenuity of yall but rather building something for the road carrying that much weight is just a big liability. If I can recommend one thing look at putting some kind of "torsion tube" down the middle of the trailer like what i think Ed's has.May help the rigidity.