This isn't really a place for relationship advice, but to frame it in the context of a startup, if a founder just won't talk about a decision you both need to make together, that's a huge red flag that maybe going into business with this person isn't the best idea.

I haven't gone through this myself, but I've read that part of picking a great founder is finding someone you can talk through issues with, even if they're uncomfortable.

Not talking about something is a much bigger issue than disagreeing about something.