Increase Wi-Fi Roaming Aggressiveness on Samsung Dart?
Even if the other AP connected, you will lose your call. When you switch access points, you are going to drop the wi-fi call. 

The device may have a hard time figuring out if it should switch to AP2 because it is still connected to AP1 and it can still see AP1. if the signals overlap, it will connect to the strongest one at the time, if it is already connected, it will not attempt to connect to another AP until it is no longer connected. Once you go completely out of range of AP1, the device scans and sees AP2, then it connects. It doesn't matter how "strong" the signal is for another AP, if you are connected, it will not see another AP. Then once you disconnect, the device needs to go in to "scan" mode. 

If your router supports it, you could turn down the power of the AP so they don't overlap (or if they do, it is only slightly). but then again, if your router supports that, you could also increase the power so you don't need the second AP.