pfSense 1.2.3 only supports PPPoE on WAN. The 2.0 beta can do as many PPPoE interfaces as you like.
If you must use 1.2.3, you can put some kind of small NAT device in between pfSense and the modem, or if you have a modem/router, see if it can handle the PPPoE login portion (often called a half-bridge).