When to add a second brood box with a flow hive? Missouri, USA

Yes. That is exactly what you should do, but only add the second brood box when your first is bursting with bees. I don’t want to type it all again today, but look half way down this answer that I wrote earlier today:

Yes, but only when the first box is 80% full, bursting with bees etc…

You will find it hard to get another nuc without a queen. People will look at you funny. :smile: Just use an extra deep box of empty (or wax foundation) frames.