You might have a requirement where you need multiple CA Directory & DXmanager instances on a single host (using different service accounts). Can CA suggest if this is possible and whether we need to take care of any specific changes that would run into any conflicts. You might be planning to use separate ports for installing DSAs for each instance.
Unfortunately, the answer is no.
One cannot have multiple instances of CA Directory (regardless of version 12.0, 12.5, 12.6 and the latest upcoming 14.0) running on the same host.
One host -> One instance.
As far as DXmanager goes, same as above. Actually, it goes further.
Even if you have two separate instances of DXmanager running on two separate hosts, you cannot manage one single backbone (environment) from two instances of DXmanager.
One DXmanager -> One Backbone.
Also to note, DXmanager is not failover capable.