No overlap/sharing between MCs. Each MC gets their own set of LIs, exclusive to them only. I apologize for the choice of word there, I'm set to update the description to make things more explicit with the release of Episode 2.
It's monogamous, so only one LI per MC. But the way the acts in the game are structured, it's fairly easy to play the other routes (you don't necessarily have to start from the beginning, just before the choice.)
But as of Episode 1, you don't need to worry about locking in. Just get to know the LIs for now.