Yeah, what @philip said.
You would have to make your own protocol (ie, chart) to send and receive data over the serial ports from the XBEE’s.
You could do something like send tables of data from the remote PAC S to the head end, then link pac display to that table, in effect having data from both.
As for pac control over the serial. No. I can’t begin to imagine what sort of comm handle chart would take care of that!
Because the serial and Ethernet are so different, you could easily run both radios/charts and not worry about interfering.
EDIT. Honestly, thinking about it, it might be better/quicker/cheaper/faster to upgrade your Ethernet RF side of things to some Mikrotik or Ubiquity gear…