We have a small office with a large number of wireless devices (a mixture of desktop machines, laptops, and wifi-enabled phones) all working from a single Apple Airport Extreme which extends our wired network. I've added another Airport Extreme for resiliency, since we've been seeing a decrease in performance and (as far as I understand) access points can only handle a small number of clients.

I set the new AP to extend the current network so that the clients weren't constantly switching between different wireless networks, however as soon as this AP was configured all the wireless devices started seeing network trouble, flicking on and off.

I'm assuming that this is because both APs are reasonably strong, and the client can't decide which to use.

What is the best route to follow to resolve this? What I'm aiming for is wireless resiliency; preferably having two APs share the network load, or if this isn't an option then having a primary AP with a "fail-over", should the primary go down for any reason.