One alternative solution for multi-device charging that avoids the overlapping coil approach that apple is reportedly having issues with is to simply move the coil to the middle of the device(s). That was the solution that Panasonic used for the QE-TM101 [0]. This particular implementation has only one coil so multiple devices are charged in series.

[0]: https://www.youtube.com/watch?v=wpSHJwiDfv4 (Not my video. Shows a Nexus 4 being placed in worst-case positions and an illuminated robotic primary charging coil attempting to couple with the secondary on the phone.)