Licensing question: failover system

[n0gnus]

Hi, I have isymphony running on our primary pbx, but need to bring it down for maintenance/testing for a period of a few days. This requires me to cut over to our secondary pbx, which I need to run isymphony on as well (our receptionists do not want to go without it). I've received temporary licenses before, but instead of doing that how could I get a 'permanent' license for both servers, but only run one at a time? Would I have to buy a full version?

Thanks!