Setting up multiple wireless access points on same network
The last time I did this, I setup the same SSID and WPA keys on two access points and set them to different channels. I've been told that due to overlapping frequencies, the channels 1, 6, and 11 are the best ones to use. I set mine up on 1 and 6. So far everything seems to be working well. My Windows-based clients connect to either one seamlessly, and the software will normally choose the one with the strongest signal automatically.
The only hitch is if a client connects to one access point, and then moves to a place where the other access point is stronger the client won't automatically switch to the new access point. To accomplish that the client needs to disconnect and reconnect. Of course, if the signal becomes too weak, that will happen automatically when the weaker signal is lost.
You should check out the Ruckus mesh WLAN products, they provide a true mesh WLAN for SMB without the enterprise cost.