The following is for cable connection between routers.
Do internet search for "connect one router to another". There's an article in wikihow.com.
You will already have part 1 done with your normal setup.
Part 2 is setup for router #2.
- disable dhcp
- assign a new address for #2; say 192.168.1.2.
- set SSID and passkey and channel to same values as #1
One more thing on #1. You have to make sure that the ip address of #2 is not handed out by #1.
Check "Reserve LAN IP Addresses".
You make an entry here to reserve ip address 192.168.1.2 for mac address of #2.
Since both SSIDs are same, a wifi device will pass between each coverage area without dropping out.