There are a variety of ways that this can be accomplished - Hardware solutions, software/driver solutions, switch trunking, etc.
There are instances where mult-interface servers make sense - In the enterprise, where there's redundant switches or your server needs more bandwidth than is available on a single link.
In reality, for the average home or small office user, there's zero benefit for linking two NIC's together. In fact, there are some distinct disadvantages of doing so.
Most people have a broadband connection to the Internet. While that might sound impressive, reality is that broadband really tops out at about 3Mb/s, 1/33nd of your 100BaseT link. Most LAN games are designed to run quite happily at about 128Kb/s. Some newer ones need more (256Kb/s) but not beyond that. - 1/400th the speed of a 100BaseT card.
Adding a 2nd NIC has two main disadvantages - It adds a tiny bit of latency since the software or (a fraction of a milisecond, but it could make the difference between fragging and being fragged), and you're making your system a LOT more complex. You might be able to get it working intially, but it could cause a lot of problems and headaches down the road when something breaks and you have to troubleshoot it. Nobody will know your config and nobody will be able to help.
This is one of those "kitchen sink" scenarios. Can you? Probably so, assuming you can figure it out. Is there any reason to do so? Nope. Is there a reason NOT to? Yup.
Why bother? It might sound cool, but it's really not going to buy you anything, and will just end up being a hassle.
- G