I believe within the Event Commands, the "Change Party Members" would be what you're looking for (under Party). You can remove your main and add the secondary character to the party, meaning you'll play as that character instead.
Yes. One of the games that I am working on starts with an empty party. During the starting cut scene the player chooses a Knight (out of two choices), then chooses 3 other party members from a choice of 12. As stated above, the "Change Party Members" event command works a treat. The only thing to bear in mind is not to tick the "initialize" box when adding a party member unless you want them to start as new characters, for instance, when you add them to the party for the first time.
Survivalist: I going to Test you: How to make the Fire. F.Mage: That easy. (Preparing the Spell) EXPLOSIVE!!!
(an Explosion on the Wood Branch then Fire is Appeared) F.Mage: (Proud Face) See? Survivalist: (Angry Tone) MAGIC ISN'T ALLOWED!!!