Hello Peter,
I am not 100% sure about being able to use the same username for more than one simultaneously but you may want to try and set it up as a guest that doesn't require username/password login. You would normally just click "accept terms" and that should log someone in to have access.
"Guest—The user does not need to be authenticated by a database" Is a quote from the admin guide but I haven't had the chance to get the product yet to play around with those settings.
I hope that helps you out and you may want to check out the emulator or the admin guide to help you out further but hopefully someone will post that has done this configuration.
Emulator:
http://www.cisco.com/assets/sol/sb/wap321_sps/main.html
Admin Guide:
http://www.cisco.com/en/US/docs/wireless/access_point/csbap/wap121/administration/guide/WAP121_321_AG_en.pdf
Thanks,
Clayton Sill