I believe you have several options. You could configure the RTK2B receiver to feed RTKBase via USB and another set of messages to the xbee header and have the radio attached there.
You could also use the normal RTKBase configuration and attach the XLR radio to an Xbee USB carrier (I assume it can provide enough power for the XLR radio). RTKBase would feed RTK corrections via the serial output.
The latter would allow a messages like RTCM 1008 through the radio interface too. Not possible if the radio was on the xbee header.