I don't like
Mary Emma, but
Mary Emily or
Emily Mary are okay.
Mary and
Emma are just not go together well in my head- they feel like different flavour of name.
Mary is old, but still good, and
Emma is modern (at least this is how I see this).
Emily is more of all time name, at least it's many decades.
Mariemma looks bad. Maryemma looks better, or Mariema, Maryema. I think of these Mariema looks best- but it may be confused as elaborated
Mariam.