I think I get the issue why it's not just as simple as turning on collisions. With aircraft, they're both moving, and by the Hitech comment I saw the other day, when they take damage, they still pass through each other, but due to the damage their trajectories alter in wildly unpredictable ways. This means they collide, pass through one another, the damage gets done, and they part ways as they fall to earth.
With GVs on the ground, they would pass through each other and take the damage, and now you've got two GVs stacked inside one another because they've got no place to fall to.
It would seem to me that if this is the reason why, the equitable solution would be to just have both of them explode. Not quite realistic, but if you didn't do it that way you'd have people running around constantly trying to drive over people in their Tigers. Also, frankly if you collide with another tank, you done messed up anyways and deserve the death.
Wiley.
