how does xbee pro 868 handle multiple end devices?

our system setup consists of two end-devices and one coordinator, programmed in AT mode. Both end-users are supposed to send measurement data to the coordinator, but that does not work. we are using the xbee-pro 868 devices.

Are there any special requirements for the PAN settings?

or, boud-rate?

a reply is appreciated!

best, Alexander.