Wifi and Zigbee (xbee) with one Arduino

Hello everyone
I have one Arduino uno with the wifi shield. I need connect this arduino with another arduino wireless.
I think I need insert too the xbee shield to communicate via zigbee.
Do you know if I can use togheter these two shields?
Have you some other way?
Thanks a lot.

Then I need to create a wifi-Zigbee gateway.

i want develop a system that permit connect the arduino to a PC and an iPhone at the same time. In other words, i don’t want any physical connection, only wireless. I’m not sure if is possible because i think that 802.15 and 802.11 work in diferentes frequences.
For exemple, create a small program in android to re-programing arduino, by wireless, not bluetooth.

Can you help me?