We have a client that currently uses a HA pair of C5210 controllers on V9 code to support the older 36xx and 37xx AP's.
They then have another HA C5210 pair of controllers on V10 code for supporting 38xx and 39xx.
Can we configure the 2 HA pairs with different code (V9 and V10) as one mobility domain to allow for roaming across the AP's?
your mentioned setup is in my opinion the best migration setup.
We have already successfully used this multiple times.
A good approach is to have in one location only with one Controller version. E. g. one side on V9 and another on V10 or one floor with V9 and another with v10.
If you mix AP from V9 and V10 controllers in an area and you have problems it is hard the find out if the problems caused by V9 or V10, in addition it's hard to track moving clients on there way because you have to look on both controller pairs (V9 and V10) in case of failures.