Another modem is not the answer. You need another wireless router or an access point. I suspect you meant a wireless router when you said you had another modem.
You can use a second wireless router by doing the following:
1) Find an unused address in the subnet provided by the current router. It should be outside the range of addresses that the current router uses for its DHCP pool. Adjust the DHCP address range if there are no free addresses.
2) Set the second router on the address found in step 1.
3) Turn off the DHCP service in the second router.
4) Connect the two routers together - PC/LAN port to PC/LAN port. The WAN/Internet/ADSL port on the new router is not used. You might need a cross-over cable or adapter, but try a normal Ethernet cable first.
Alternatively, take a look at Powerline network adapters such as:
http://www.tp-link.us/products/details/?categoryid=2244&model=TL-WPA4220KIT
This will avoid running long Ethernet cables round your home.
I hope this helps.