christcorp,

You are correct. If two ATAs are behind your router are using port 5060, then your router maps one of the ATAs to something different than 5060 on the public side. But on the way in, if the data is sent to the correct public port, it will reach the correct ATA.

Like you said, if you are doing port forwarding, you should be aware of this. Since the OP was using the term "PAT" (port address transalation, which is what we all have and not NAT), I think the OP is familiar with what they are doing.

Also, when you use multiple ATAs, since the port numbers could be different on the public side, in many cases you probably need to enable STUN. STUN will figure out correct public IP and port and use that in the SIP messages. Sometimes the routers might change the IP/port in the SIP messages (I think that was ALG does). I think that's the reason some combination of routers/ALG/STUN don't work well.