I just did this myself, works great. Set up the two wireless networks normally under Wireless/Basic Settings. Then go to Wirelss/VLAN and QoS. Enable VLAN and click OK on the popup message.Under VLAN ID of each network, set the correct VLAN. Then, change Default VLAN ID and AP Management VLAN to your main (non guest) VLAN. (This may only be required in one section, but after a lot of testing it was best to set both of these). Select tagged under VLAN Tag. This is all that is needed. I also have that port on my switch set up as:
switchport trunk encapsulation dot1q
switchport mode trunk