I'm planning on mounting 3 on teh floor (both sides and down teh middle).
When I got my trailer and bought my e-track, that's what I was planning to do to. You might want to give it a little more thought before you do that. I ended up going with a strip down each side and 4 flush-mount D-rings in the floor. 2 about 8' from the back of the trailer, and 2 right at the back of the trailer. When I've got 2 bikes in the back of the trailer, each bike is secured at 4 points. The two outside straps hook to the e-track. The 2 inside hook to 2 of the d-rings. Thus, each d-ring only holds 1/4 of one bike.
If you put the e-track in the middle, you may have a couple of problems One is, if you put one strip down the middle, it won't be far enough out from the side of the bike you're hooking to it, to give the bike good support. The tie-down will be pulling almost straight down, instead of down and out. At least, that was the case in my 6' wide trailer. So you end up needing two strips down the middle. One just to the center side of each bike's wheels, that you hook the OTHER bike to.
But then, with one or two strips of e-track in the middle of the trailer, moving anything around on the floor of your trailer that has to roll or slide becomes a real problem. For me, I have a rolling tool box that I wheel in and out of the trailer. And I have a rolling mechanic's stool that I sit on when I'm working on my bike. In either case, if I'm doing that in my trailer, having that e-track in the middle of the floor is a real pain.
Just food for thought.