Typically, yes, you can do this. Keep in mind that your router must be able to open ports to a range of IPs and not just one specific IP where ports must be used by multiple devices. Even on the same port, it should be able to route to the right private IP correctly with an open SIP session. If you are using SIP for both, then there will definitely be some shared ports, but there should be no conflicts. If both phones will be set to connect for their TFTP information separately and know where their gateways are, then there will be no issues with the router as far as getting them to register and initiate/receive calls.
You may still encounter some issues with prioritization and saturation, depending on your circuit speed and routes for the calls. I highly recommend also disabling anything to do with VOIP in your router as it will cause nothing but conflicts with what you are trying to do. It is also prudent to disable some firewalling aspects as well. Basically anything that is labeled as ALG, VoIP, Intrusion Detection and Stateful Packet Inspection should all be disabled.