I can only offer analogous info as I don't own any trinities, but if the sub is well made they can generally hold a lot of weight. For example, people have stacked SVS PB13 Ultra's three high, and they weigh 155 pounds each (so 310 pounds stacked on top of the bottom sub). I've also used my Ultra's and my old PB12 Pluses as impromptu step stools to adjust my rear surrounds on occasion, and I weigh 260 pounds. I haven't heard of anyone having any issues and the sub looks pretty well put together, so I'd *guess* that it should support the weight of another Trinity stacked on top. Of course, that is just a guess.
Have you tried asking Definitive if they have any positions on the idea?
"Every loaf of bread is a tragic tale of grains that could've become beer.......but didn't...."
"If it works the first time, it's not high end." ~ Harry Pearson