So I've tested it at home, where I have 2 different APs on 2 different channels (11 and 6, if that's of any consequence). I purposely rebooted the APs the RPi was using for internet access and it seamlessly switched over to the other AP without killing my laptop's connection to its own AP. I then rebooted the AP it was connected to after the first one had come back up, and it switched back again.
While this seems to behave sanely, you may run into issues where your device will drop session with the RPi AP due to "no internet connection". Assuming you're running Android, your device may ask if you want to remain connected to that AP, or it might just disconnect due to lack of internet. That's entirely device-dependent, but there's nothing inherently making this not work based on my instructions. It just might take some extra work on the device-side to make sure it doesn't try to drop the connection.
While this seems to behave sanely, you may run into issues where your device will drop session with the RPi AP due to "no internet connection". Assuming you're running Android, your device may ask if you want to remain connected to that AP, or it might just disconnect due to lack of internet. That's entirely device-dependent, but there's nothing inherently making this not work based on my instructions. It just might take some extra work on the device-side to make sure it doesn't try to drop the connection.