say A NAS use 11021 and B NAS use 12021
and I would also set A NAS use IP address 192.168.0.110 and B NAS use IP address 192.168.0.120
you should do all IP management and port management first then nothing is trouble.
also port forward set to 11021 forward to 192.168.0.110
and 12021 forward to 192.168.0.120
or even config the FTP port to be port 11021 on NAS end with IP 110 also then no need to think.
just like all my home devices uses different range of ports to BT.