It's UNIX, so there must be some way of doing it, but by default, OS X uses only one network interface at a time. As soon as the AOL goes up, it becomes the active network interface.
I'd suggest you get some real internet connection for your network, and either ditch AOL altogether, or use AOL's bring-your-own-connection service which connects over your existing internet connection.
Apple's Airport base stations with modem allow you to share an AOL dialup connection onto a network, by the way.
tooki