Same SSID on two different controllers: Bad idea.
1: They will try to manage the airtime and will register 'rougue' AP's broadcasting the same SSID.
2: You will have different DHCP servers on each net, and what about registration? WPA or Captive? If some sort of portal is in place, you will have guests registering on two different portals, and That is gonna be a strange user experience!
Make an event SSID, or make sure the DHCP/portal is at least the same.