This depends on the setup of your cluster.
Normally there is just one master server, if you attach another server to this master, then this additional server amy not run a ispconfig interface. If you would install a interfacethere, then any change you do there will cause a conflict with changes that you do in the master server and the system will break sooner or later. An example for such a setup is this guide:
If you setup the two servers as mirror system like it is desctibed in thsi guide:
Then you will be able o use the ispconfig interface on both servers because the ispconfig database of the first and second server are mirrored with mysql master/master replication. So changes get synced in both directions.