Unfortunately you cannot use a conventional Modem/Router with your 3G USB dongle.
What you can do is to have one computer online via the dongle, connect the two Laptops (either with a *crossover* ethernet cable or in an Ad Hoc wireless network) and enable "Internet Connection Sharing" on the 3G connection. The second Laptop would then have internet access provided the "host" Laptop was running and online.
I've written tutorials on setting up an Ad Hoc wireless network with Vista click here and XP click here
Bear in mind that performance with two computers connecting simultaneously via a 3G service isn't going to be very spectacular. ;)