Windows 3.1 has the problem that you can't have two network cards of the same model installed, which can be an issue with Microsoft Virtual PC. But there's a trick to use two virtual network cards within Microsoft Virtual PC:

1. Download the Intel drivers from http://www.intel.com/design/network/drivers/Nd2_300.htm
2. Download the DEC drivers (search Google for GF10011.EXE)
3. Boot Windows 3.1
4. Go to Network Setup
5. Go to Adapter
6. Install the Intel drivers for the first adapter
7. Install the DEC drivers for the other adapter
8. Configure both as desired

For some reason, both drivers work just fine with the network card Virtual PC emulates, and they won't conflict with each other at all. This is really useful if you have some complicated networks like I do.

An example of when you'd have to use two network cards?

My router is kind of screwy, so I have it set up so the first network card goes to the VPC's NAT and the second network card points to the loopback adapter. Everyone can communicate with each other and all virtual computers can still go to the Internet. With the router, I'd get strange disconnects because it can't handle the virtual PCs properly, for whatever reason.