If you have multiple AP platforms in a cluster, you can only update via anonymous ftp, tftp (probably best), and http. You would have to stand up a tftp server and load firmware for both platforms and then put the tftp path for both sets of firmware for the upgrade. You cannot just upload the files to the controller like when you had a single platform cluster.
The best reason is that multiple AP firmware cannot be stored on the VC, so access points on each platform are pointed to the URL where they are all updated at the same time. If you only had a single platform, you could upload it to the VC, and the VC would update the others. Since you have a multiple platform, all APs are pointed to your ftp or tftp or http URL, where they will all update directly.
I hope that makes sense.