1) Turn of DHCP on the 2nd router.
2) Change local IP address of the 2nd router. I would say to 192.168.1.254 to make it easy. If you don't do this, after you turn off DHCP, you'll have no way of managing the router.
3) Change Wireless settings
Voila, done. Everything will be on the same, happy network. You can even turn on wireless downstairs. Just name one downstairs and one upstairs, it will work just fine.
Edit: You may need to unplug both routers and plug them back in for all the network changes to take place. Sometimes they will hold on to IP addresses and assign the same IP address based on a stored MAC address. In this case, you definitely don't want that. Unplugging both units for at least 30 seconds will do the trick. I would turn on Router 1 first, then Router 2.
