Hello,
I'm using two RTL8139C NIC with the same driver without your problem.
Oups !
I'm tired! I'm using SME 5.6 on this server (...) Sorry.
No problem with two Intel Pro 100 NIC on SEM 5.6 too.
May be it's your specific adapters. Can you try two others "identical" cards like RTL8139, Intel pro 100 or others ?
I have a question : When you said "I use the same driver for both", it's under Admin (blue & grey ASCII interface), you obtain txo identical lines about drivers for your NIC.
I don't know why but I use first line for the first NIC (required for LAN) and second line for second NIC (for PPPoE).