Stock firmware probably can do it. Not used it in years.

If you want the APs to still act as APs for other WiFi clients then you need to use WDS to connect the two. You must use WPA and not WPA2 when you do this as it doesn't work if you do.

For WDS:

Set up both to have the same SSID, same channel, same WPA keys and type in the other APs MAC address into each ones WDS settings page. Thats about it...