Good catch Dirk67, but that's not exactly the case. It looks like that in access point mode, only one client AT A TIME is supported. Refer to:
http://www.atmel.com/Images/Atmel-42420-WINC1500-Software-Design-Guide_UserGuide.pdf
Page 17 is the state chart. After the AP has completed its business with one client it can take the branch m2m_wifi_disable_ap and then ready itself to listen for another client or even transition to one of the other modes. The example on page 49 uses an endless loop but it could just as well terminate after business is completed.
This is of course based on the existing Atmel Firmware version 19.3.0 but that is of course subject to modification or improvement in the future.
However, this begs the real question of whether or not the community will modify the Arduino library to support it. That could be a significant effort, and it might be better to use an off-the-shelf WiFi access point and be done with it.