Actually it would require more than just a second Atmos capable receiver since they can't communicate with each other about speaker -vs- sound positioning.
There is a large and detailed thread about this topic in AVS, and currently without going to something like the Trinov, the only way to expand beyond 7.1.4 is by matrixing the other positions, so they would not be discrete.
FYI, BD & UHDBD actually has which ever mix the studio decided to place on it, up to the full 24.1.10 cinema version, it all depends on the receiver as to the number of speakers used.