How can I set the priority of Wi-Fi networks on my iOS device? I have two Wi-Fi networks in my house, one at one end of the house and one at the other.  While signals crossing from one side of the house are usually audible on the other side, they are generally fairly weak and quite nearly unusable - something in the house's construction attenuates signals traveling through certain walls.  So, to enable good connectivity while roaming the house, I have to have my Wi-Fi clients configured for both networks.

This works fine on PCs where I can configure both networks and set a priority order, so that the system prefers the network closest to its normal location and should only roam when its usual network gets too weak.  However, I cannot find a way to set this priority in iOS.  This results in some devices occasionally connecting to the farther AP, despite being in their regular spot where they should prefer the closer one.

How can I customize the roaming preferences for Wi-Fi networks on the iOS devices?  I at least want to assign a "preferred" network for each, but would also like to know if there's a way to set a "roaming threshold" - a point at which the device should choose to change APs, versus remaining on the current one.

I'm pretty sure all my iOS devices are running iOS 5 or higher.
I'm pretty sure this is not possible to do on vanilla iOS, perhaps with a jailbreak.

However, your solution of running two separate WiFi networks is generally not what you want. It is much more common to extend one WiFi network (i.e. a single SSID) with multiple routers. Devices should then automatically switch to the access point with the stronger signal.

You can check this question which describes how to setup two APs for the same network (i.e. a single SSID).