In theory it's fairly simple to do actually. Make sure all the DHCP crap is turned off on the 2nd router and that it's IP address isn't the same as the other but is in the same range.

To make a wireless bridge between the two of them you then have to set the SSID, channel etc in the wireless settings to the same as the other router. Then setup WDS on both the routers so they'll bounce the link between each other. (download the router's manuals for how to set that up)

Of course it has it's downside, half the wireless bandwith will be eaten by the link.