Your answer was brilliant. Your detialed reply give me a new knowledge to the DNS and lms-ip.
Actually,a thought suddenly came out in my mind.
I can do this,the two acs have the vrrp configuration,master one is the vrrp master,local one is the vrrp local.
These APs registered to the VRRP virtual IP.I can shutdown the master one's vrrp interface.
Thses APs still register to the the virtual ip,but the virtual ip is taken over by the local one.
The APs resigtered to the local one.
I know this approach is limited.It just adapt to the local and master are in the same subnet.
Ohterwise,I should consider the DNS and LMS-ip.
Whatever, thanks for your detialed reply.