When you covert the pair into SSO, all the APs will go to the ACTIVE unit. No unit will "live" in the standby unit because this unit will "share" the AP-support license between the two.
This is the first step you need to get sorted. Send an email to email@example.com. and give them the exact details of what you want to do (i. e. AP SSO) and then provide the serial number of your nominated active WLC and the serial number of your nominated standby WLC.
Ok, but imagine both wlc before convert in ha sso, wlc1 has 1 ssid called sales and wlc2 has one ssid called workers, when you pair both wlc in ha sso whats going to happen with the configurations of both WLC, are going to be merged, or you are going to lose the parameters of one WLC and only one configuration is going to remain ????