Let's suppose we have 2 objects who are master objects (P1, an P2) each of them have two detail objects, like this:
P1 is master for C and C1
P2 is master for C and C2
As you can see, C is a junction object (2 M-D, one for P1 and one for P2). For purpose of these exercise P1 and P2 have OWD private, and I have unchecked "Grant Access Using Hierarchies" to disable the role hierarchy for these objects.
To keep it simple, I have created records that have the same name as the object (P1, P2, C1, C2, C) and these are the record access settings:
Notice that Melisa has access to all records, and she is the only one who can see the C record ! In order to see a record with 2 Master-Detail relationships, you must have access to BOTH parents. But the C record will be deleted when any (P1 or P2) record is deleted.