People in here are really overthinking things about MC.
Guys, he's someone that's not accustomed to setting boundaries. I could see him being this inept at scheduling even with male friends. He's not trying to string either along, he is just out of his depth with both girls.
he cannot decide which one to date
He's romantically interested in both in the sense that he is attracted to both (which makes sense, as he they are attractive girls). Unfortunately, as a virgin, he lacks (and knows he lacks) the experience being around girls to know what is friendly banter (as opposed to flirting) and how to set boundaries with girls.
This is not malice, and it does not help that Azusa is all but openly mocking his virginity, throwing off his frame of reference when it comes to another outlier like Ametella.
he could have scheduled better
He doesn't want to disappoint either, because he clearly lacks friends in general, not just female ones. Again, if these were two guys, he probably would still fumble into a double-booking due to not wanting to have Ametello feel alone or Azutaro to become bored of him.
Add on him not knowing if there was a possibility of one becoming a girlfriend or not and of course he's not making the objectively best decisions. He doesn't even know that Ametella is a yandere.
Again, he did what he could, and Azusa threw a wrench in everything by showing up outside of the scheduled time (which goes against those saying "just set boundaries" because she clearly broke an existing agreement, but one so minor that barring her entry would have been the unreasonable action).
The point of a drama is that someone cannot handle a situation optimally due to a flaw. Anger issues, greed, over-ambition, etc were the flaws in Greek tragedies. Here, it is inexperience and a desire to please everyone involved. Unlike us, he doesn't have the benefit of seeing the entire story, or to pause for 15 minutes to think up a solution between turning a page.