There is a little problem with this:
If you have a rad on the floor, it should be sucking air into the case. And the top one will be pushing air out of the case. This is the equivalent of sandwiching rads and see here how that's not good:
Basically, the bottom rad gets cool ambient air, but the top rad gets hot air warmed up by the first rad and the case components. So the second rad is almost useless.
This I why I like the bracket I recommended to you earlier. With that, you get fresh air for all your rads.