What I imagine that you mean is that you have 2x6 joists running left to right (in the picture in your first post). The ends are notched into the web of the beam that we see in the middle, with the other ends attached somehow out of the left and right sides of the picture. The decking runs top to bottom, screwed down to the joists. Maybe you have blocking stiffening it all.
And this is because, referring to Mike's post #4, there in fact is NOT room to lay 2x sleepers on top of the grid. Is that the case?
If so, I'd spend some effort figuring out how to keep the connection snug, not to have the gravity and bouncing pull the Trex and the joists apart. On a normal deck, the force of gravity doesn't try to pull the screws out. In this situation, the force of gravity (and bouncing, etc.) is going to try to pull the decking and the joists apart, possibly loosening the screws by pulling them down into the Trex. This could be tough to manage especially as moisture comes and goes, and the joists expand and shrink. I'm not sure if that would be a problem, or how to deal with it.
Edit: I'll bet there are clips for attaching stuff to grid. Maybe use from below to keep the joists snug to the metal?