Unfortunately, IPCOP does not support multiple GREEN interfaces. It has been muted that support for this may be introduced in future versions, but don't hold your breath. As you may already know, IPCOP allows up to 4 interfaces:
1. GREEN — Internal Trusted Network
2. BLUE — Wireless Semi-Trusted Network
3. ORANGE — DMZ
4. RED — Internet
The BLUE can be used as a second GREEN interface with a few modifications, but that's really it.
If you're after a specialised *nix firewall distribution which supports multiple interfaces, check out pfSense
which is a fork from m0n0wall
and is based on FreeBSD
. It supports up to 32 interfaces (including 2 WAN) which should more than meet your needs!! It is however still in Beta, although it looks impressive and is one project I will be monitoring.