These days knowing how poorly the Central box makes use of Rx2 I've been thinking I'd prefer to use an REX7, etc as the primary with an R3 secondary (or clone) connected directly to the primary rx via a PPM link rather than connecting the secondary to RX2 of the central box. You should then be able to connect a 900Mhz receiver to the RX2 input so you would have 2, 2.4Ghz receivers in a configuration that I believe works better (as far as we know at this point) than the secondary directly connected to Rx2. Since Rx2 never gets used unless rx1 has almost completely failed it seems like a perfect match for the 900Mhz backup which also doesn't get used unless 2.4Ghz communications fail..