I've never done it, but it looks like the capacity is there. Go into the Network Connections and add a new one. In the Advanced tab, you can "accept incoming connections". I don't have a modem, so I can't go any further, but the description says you can do so via modem.
One note - V.92 is really a usability enhancement of v.90. There *are* some better compression routines and a fe wnew goodies that let you hit about 48K when both ends are analog, so it's better than the 33.6 than V.90, but don't expect massive improvements. Keep in mind that true 56K connections require that you have a digital line on the receiving end of the call, like most ISP's do.
From here, you're on your own. You'll probably have to run ICS or some Internet sharing software on your main computer to allow the secondary one to get out, but I'm not sure what kind of connection it makes via the modems - Most likely a PPP, but you'll just have to experiment. If it asks you anything you don't know what to do with, let me know.
- G