Even though this is not explicitly denied in the manual, this can not be done.
The RS-485 port on the FP-1000 module is a repeater port only. The FP-1000 does not look for commands addressing it on the RS-485 port so you can not use two FP-1000's on the same serial port. You can use one FP-1000 and up to 24 FP-1001's (with the FP-1001's hooked to the FP-1000's RS-485 port) or if you have a RS-485 port on the computer you can use up to 25 FP-1001's per RS-485 port.
Regards,
Aaron
LabVIEW Champion, CLA, CPI