Zigbee low power mode when both devices are battery-powered
In ZigBee there is always a Coordinator that starts the network. There can optionally be Routers as well (the Coordinator is a Router as well). The Coordinator and Routers are always on (typically mains powered) devices. LowPower devices are called EndDevices and they can sleep. EndDevices need the Coordinator or a Router as the "Parent", the Parent queues messages for their EndDevice "Children". 

So there is no synchronization between 2 EndDevices that are communicating. They send the message to the other EndDevice, which is queued by its parent, when they wake up they "poll" the parent to see if there are any messages queued. 

You can use one of these SDK's:
http://www.ti.com/tool/z-stack

And this packet sniffer:
http://www.ti.com/tool/packet-sniffer

To test the behavior.