Matt, I'm not sure what the limitations are. We have multiple ethernet cards in our 400s and are able to route specific IP traffic through them. For example, we have one ethernet card dedicated for sending 400 to 400 mirroring traffic, and another ethernet card for other lan traffic.
400 TO 400 Mirroring
In our situation, AS4001 has line ETHLIN01 using IP Address 192.168.0.1 to speak to AS4002 using line ETHLIN01 with IP Address 192.168.0.2. We have a route on AS4001 that looks like:
RTDEST(192.168.0.0) SBNMASK(255.255.255.0) NXTHOP(192.168.0.2) PRFIFC(192.168.0.1)
and a route on AS4002 that looks like:
RTDEST(192.168.0.0) SBNMASK(255.255.255.0) NXTHOP(192.168.0.1) PRFIFC(192.168.0.2)
Other LAN traffic routing through other card
AS4001 with ETHLIN02 using IP Address 172.30.127.157 communicating to AS4002 with ETHLIN02 using IP Address 172.30.127.158. We have a route on AS4001 that looks like:
RTDEST(172.30.127.0) SBNMASK(255.255.255.0) NXTHOP(172.30.127.158) PRFIFC(172.30.127.157)
and a route on AS4002 that looks like:
RTDEST(172.30.127.0) SBNMASK(255.255.255.0) NXTHOP(172.30.127.157) PRFIFC(172.30.127.158)