two networks wants to connect to proxy f

i am having two diffrent networks in the concern, in a particular network i had connected a modem, a proxy is also enabled so that i can share the web connection in that particular network, i had connected the modem in a client machine. now my problem is i want to share the web connection using the proxy server in the other network also, note that i am having only one network card, tell me how can i do this, what r the procedures.