Connect to Network and AT&T 3G Service

I have a wireless network setup at my father's business so that all computers can access a server and run the business point-of-sale/accounting software. It's located in a rural area so internet has been done through dial-up as needed on that particular computer. We decided to give AT&T's 3G service a try and only need it to work on one of the computers right now. Is it possible to have that computer connect to the network wirelessly through its USB adapter (as we've been doing) but also simultaneously connect to the AT&T service through the USB adapter provided from them? Right now, connecting to one automatically kills the connection to the other. The computer that we want to connect is running Windows XP Home. Basically I need to make two network connections simultaneously with two different devices. One is simply a network without an internet connection and the other is the internet connection. Thanks!

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