Are the existing controllers master:local at each site or local:local ?
To accomplish a split of the site you will need to have two AP-groups in play
AP-group-1 . LMS-IP = Site Controller 1, BKUP-LMS-IP = Site Controller 2
AP-group-2 . LMS-IP = Site Controller 2, BKUP-LMS-IP = Site Controller 1
Then you need to reprovision APs, half into AP-group-1 and half into AP-group-2.
JF
PS - This adds comlexity in troubleshooting and roaming if the boundaries between the APs on the different controllers are commonly/frequently crossed by clients. Just keep that in mind.