Timing the renting out of primary to move back into rental

[Jack B.]

I live in a house that has a MIL apartment with it's own bath, laundry, kitchen, bedroom, living room and dining room above the garage. I live in the main part of the house. Not sure how easy it would be to rent out the main house while having the apartment still rented to someone else.

I have a rental that I'm considering moving back into, but would like to time it in such a way that I don't have the people in the rental move out too early, leaving me with the cost of the mortgage, and even worse, I'm not having them move out early while my primary ends up not being rented out yet either.

Everyone is on a month to month lease, so 20 days notice before the end of the month would be what I need to get anyone to move. 

So what's my best move here, wait until I find a tenant for my primary before I give notice to the tenants in the rental I want to move back into?

If I don't find one in time for my primary to give notice to the tenants in what is currently a rental, perhaps I setup the lease for the people who want to rent my primary for the following month when I can have the rental empty after giving proper notice?

The goal is to avoid having any rental empty.