After much agonizing, I've decided to finish the book with a single protagonist, for four reasons.

1.) My rule about not making major plot changes until I'm finished. I may find that I really like the end result, and more often than not, my original instincts are better.

2.) If I start adding the second character halfway through the book, it will necessitate changing the first half the book. Whereas if I decide not to change, I'm free of the necessity.

3.) I have a feeling that adding a character is easier than subtracting a character.

4.) If I decide to add the second protagonist, then it gives me some meaty working material for the rewrite, which I always like.