No problem with the same network, as long as they are on different servers with different IP addresses.
If both are in use, it will affect network traffic volume, but no actual conflict. We tested FMS12 while FMS11 was still running on the same network -- no problems, but did not use 12 heavily.
I can't seem to connect to the FMS12 via an outside connection (with FMgo12). the fmgo11 seems to work fine. I assume that I point fmg11 and fmg12 to the same external IP.. I had our IT guy set up port forwarding.
you will have to specify different ports for your server location for 11 and 12 and forward those specific ports to the different servers.
xx.yy.zzz.aa:4444 for FMS11 forwarded to 192.168.1.11
xx.yy.zzz.aa:5555 for FMS12 forwarded to 192.168.1.12
where xx.yy.zzz.aa is the 'outside' address of your router
Check what ports 4444 and 5555 are used for before using them specifically