Cisco won't allow more than one SSID per VLAN per Interface using Autonomous APs. Can't answer for WLCs, but I would assume the same thing.
If you have a single radio AP, I recommend having a test VLAN to go with the test SSID and then have your router route between the VLANs.
AP Message When Attempting:
Enter configuration commands, one per line. End with CNTL/Z.
(config)#dot11 ssid Example
Warning: Vlan 21 already mapped to SSID Guest. SSIDs with same vlan association cannot be attached to the same interface.
Originally I took this as one to one but when I tried it on a dual radio AP I was able to replace the Guest SSID on the 5Ghz radio:
(config-if)#no ssid Guest
#sh dot11 bssid
Interface BSSID Guest SSID
Dot11Radio0 0026.0bXX.XXXX Yes Guest
Dot11Radio1 0007.7dXX.XXXX No Example
EDIT: Corrected myself, it does work on a dual radio AP, but not single radio