Only one way I can see to do this:
Put a GB switch between the wall and both devices, give up the existing POE, and get a power supply for each phone.
There are power bricks that supply POE for one device.
Use a Cisco SG200-08P gigabit 8 port switch. Not too expensive, nice management interface (remember to save your config changes), esaily established QoS for call prioritization and ports 1 through 4 are 48 volt PoE ports. Easy peasy.