Perhaps an ethernet data switch? (Not too different from A/B switches used in audio, video, parallel port, printing, etc.)
Here's a link to an item which looks as though it handles A/B ethernet communications:
Are any of your routers/modems wireless? Can you connect to one of them via wireless, and to the other via ethernet? I believe you could communicate on both simultaneously.
It might also work to install an additional ethernet card.
I have networked my own computers (Mac and PC WinXP), and I have gotten familiar with network settings and control panels. I have only seen them recognize one ethernet port... however that might be because my computers each have only one ethernet port. There may be software that can look out beyond your port to find multiple networks, and let you choose which one to communicate with.