Hi,
I ordered several ERCs some weeks ago to be controlled over a single serial port, but haven't tested them yet. Manufacturer said it's OK if their control line "disappears" so switching ERCs with relays should be ok, it's just rxd, txd and gnd. Their card for az/el should hardware wise be OK for 2*az, but the protocol or SW not yet. Please ask ERC yourself, their support seems excellent. The control end SW, like HRD would have to accept that also. Or maybe a second set of RRCs...