Roaming wifi AP's

I set both my AP's to same SSID, diff channels, and same passphrase but my portable Android devices don't want to switch to the best signal. Worse, still, with both SSID's the same, you can't manually select the strong one as Android won't display two identical SSID's. Pity, really, as it would have been useful here with an upstairs and downstairs AP.Anyone got Android devices changing AP's?