eth1 on Nios II/ecos

I assume you're using the Altera_avalon_lan91c111 driver.

I might be wrong, but I think if you look at the .inl file that's in the ecos-current/packages/devs/sopc/*lan91c111*/current (wildcarded because I forgot the exact name) directory, you'll see that it declares an instance of the driver and hardcodes the name "eth0". If you duplicated this declaration and changed the duplicate's name to "eth1" (and figure out a way to get two different MAC addresses in there), it might work. I can't say for sure, because I haven't done it myself.