Technically, you could use a second modem, but the other modem must be disconnected. You can only use one modem at a time.
Modem/router and wired computer in one room and a wireless computer in another room is the most common way of doing this. Most wireless routers have 4 standard switch ports for wired computers in addition to the wireless section.
Here's links to a wireless modem/router combo and PCI wireless card.
http://www.ebuyer.com:80/UK/product/94178
http://www.ebuyer.com:80/UK/product/45602
If the computer you want to connect wired does not have an Ethernet adapter, here's one:
http://www.ebuyer.com:80/UK/product/25986