I have a setup with a VX9000 and three 7522 APs which are adopted by the VX9000.
In the same subnet I setup two 7532 one as VC and the other as adopted AP by the VC to test stuff.
When I rebooted the VX9000, all three 7522APs got adopted by the 7532VC !
when the VX9000 was back up it re-adopted the three 7522 but things were not working well because the 7532 had a newer firmware and upgraded thet 7522 APs, resulting version missmatch when the VX9000 re-adopted them.
Is this normal? Is there any way to prevent this from happening and have two isolated controller in the same subnet?
Yes, The best practices guide says to add it to "Profile" instead of "self" overrides. But in your particular situation were APs were adopting to the wrong wlc, adding controller host to the override profile is way to direct the AP back to correct wlc.
Best way to prevent the adopted APs form adopting to another controller is to segregate them by putting them in a different broadcast domain.