Should I use a state machine or multiple actors to handle the player?

For example in a turn based RPG or instance based RPG the player would control vastly different things, should I still couple them into one character with state machines or should I split them into 2 separate actors and spawn them accordingly?

You should split up different characters(Pawns), that way it will be easier to debug and separation of concern is always better.

You can start with a single parent Character/Pawn Base class or an Interface class to couple all the common functionality.

All you need to do to switch to the different things is just Possess and Un-possess the needed character. (See Player Controller Class)