I have two 75 GL tanks paralell to the floor joists. The front to back dimension of the stands are only ~19.25 so they only sit on one joist paralell. It would be possible with 19.25" to position them over two joists 18" apart if positioned just right. This would distribute the weight across my floor significantly better but 1) the center of both tanks are sitting directly on a beam and one tank is actually centered directly over a column (crawlspace) and 2) the floorboards are plywood, not OSB or particle board. They've been sitting steady there for ~4 years.
I'd think you'd be fine but it would be better if you could add a piece of plywood under the stand a few inches wider (front to back) so it reaches another joist. If the stand sits on two joists there's no problem.