I use to be a Comacast installer and can answer this one.(Note: left the company mid-2004 but I'm sure not much has changed on the back-end of things)
Short answer:No
Long answer:It's possible, but.....
Brief explanation: This was a common thing I heard when I did hi-speed installs. Most people think 'One modem is fast then 2 must be faster', but that's not quiet correct. People think that if they have a modem for each computer that each one as a fastest connection possible. But if you think about it, both modems hook into the same cable that feeds your home from the pole(or underground line). They would both be connected to it via a splitter in your home. The internet signal is broadcast across the cable via a certain frequency bands. For example my area used the freq's around the 729MHz band for internet use. Even though there are in reality multiple channels/bands available(we had up to 3 in my area), your mulitple modems would be trying to use the same part of the 'pipe'(freq range) at the same time. Slowing down the speed across all of them. So to get equal speed over mulitple modems, you would have to have one 'drop' (physical cable feed) per modem going to your home from the 'tap'(were your cable line connects to the main line at the pole). Most cable company networks are only designed to have one 'drop' per residence to keep overhead costs down. Also on the network side of things, you can only have one active modem on an individual customer account. I believe this is a limitation of the software they use. If you try to add multiple modems on one account, none of them will be able to get a ip address because the 'headend'(backbone of the network) doesn't know which one is suppose to be active. In theory you could get mulitiple drops if your home was setup as a MDU(multiple dwelling unit-apartment), but more then likely they would not allow it or charge you a small fortune to do it.
For most use, your not going to max out your single connection anyways. That's why home networking is the most popular way to go. Just use your Airport Express to connect both computers to the modem. If you want a faster 'wired' connection, then look into wireing your home for ethernet. Then just share your internet over a wired router.(persoanl note: make sure you get a good one. my month and a half old linksys router went out about a week ago. seocond one in a year)
Hope this wasn't too long winded. Hope this helped some.
Edit: Speed wise, the cable modem speeds(3-8Mbps depending on area) means the speed of the conection of the modem to the internet. The 802.11 speeds(example-54Mbps) means the speed of the wired/wireless router connection to your computers. Meaning that your computer to computer home network will be faster then any of their connections to the internet.
