Our ethernets are ethernet switches. These are big comm devices that link to each computer. This is a different architecture than the original ethernet where all the computers talked over the same wire. The ethernet switches have a port for each system. Each port can send and recieve at the same time if the ethernet card in the computer supports it. There is no collision because there are two data paths. We do have some servers with two cards going to the same network. Each card gets a separate ip address. But this is usually done for more bandwidth to the network. The only other reason is redundancy, if one card goes down the other takes over. But we are more likely to use redundant servers for that.

You don't need another card for multiple ip addresses. Every version of unix that I know has some way to assign multiple ip addresses to a single card.

It is possible to have different cards going to different networks. If ip forwarding is turned off, this is done for redunancy again. Turn ip forwarding on, and such a computer will act as a router. Our network folks don't like that and we are prohibited from doing that. We must use routers for our routers.