Harvey,
i suggest you replace the secondary R3/RSW with the 900 Rx, and try running the combination in single Rx mode (not dual path), and then disable the 2.4 transmissions to check that it correctly switches over to the 900 Rx.
While I've never used a 900, I assume that it is very similar to an R3, so make sure that it is set up for EX Bus output into the REX.
I don't recall, but I suspect that the REX cannot handle multiple secondary receiver inputs and then perform the necessary switching. You would need a CBxxx to do that function. I guess you could daisy-chain to a second REX instead of the R3/RSW and then connect the 900 to the secondary REX. I've never tried that though.
Paul