It takes a very highly skilled C to serve multiple roles for a couple and the individual partners.
To be effective as an MC the person has to be viewed as being on both partners' sides - the client is the M. To be effective as an IC the C is on the individual's side.
The potential for conflicts of interest are innumerable, and a conflict can really screw up an M and/or a person.
If you have any question about the effectiveness of a C, it's better to use that C in only one role.
JMO, of course.
We use the same person as MC and IC for each of us, though....