AP licenses are not required on second controller because SZ100 in a cluster uses the AP license from the available cluster AP pool.
For example, if I have 100 AP licenses mapped to SZ100-1 and later If I add another SZ100 (SZ100-2) to cluster, total capacity will remain 100 AP for this cluster. Means I can connect 100 APs to this SZ100x2 cluster.
Though you should purchase support contract for second SZ100, as future upgrade are not possible without support contract for each SZ-100.
Syamantak Omer | Community moderator | Sr.Staff TSE | CWNA | CCNA | RASZA | RICXI