Your setup can not work with ispconfig. I am the auhor of ispconfig, so you should believe me in that regard. Idpconig works with configuration trnsactions and it stores also the processing state of the local system in the dtabase, so if you connect two instances to the same db, the system can not process any changes as two processes overwrite their transaction states.
To get your setup working, undo the mirroring of the config parts as ispconfig is doing that. THen install 2 mysql instances as thats a must have for ispconfig.
You can use your mysl cluster for the website databases but not for the two dbispconfig databases, as ispconfig mirrors its database contents internally.
The setup can be loadbalanced even with one master of course, as the main reason for such a setup in most ips is for loadbalancing the sites, mail accounts etc. Loadbalancing the controlpanel is normally not nescessary if you dont expect to have more then a few hundred thousand pageviews per day from the controlpanel itself. To get that, youw would need to have several ten thousand customers on that system.