OK, thanks. This is going to be a bit of a challenge since you have a mix of an older "g" Express and a newer "n" Express. You'll need to use Apple's WDS type of setup for this, which is difficult for some users to configure. Follow the steps very carefully on Tesserax's instructions in the link below.
You'll need to connect an ethernet cable from one of the LAN <-> ports on your main router to the "main" AirPort Express. When you configure the wireless settings for this device, you'll need to use the exact same wireless network name, security and password as your main router. The final step of the setup of this device (before you click Update must be to configure it as a bridge. You will click the Internet icon and make sure that the Connection Sharing setting is set for "Off (Bridge Mode), then click Update.
http://discussions.apple.com/thread.jspa?threadID=2355832&tstart=0