We would like to move and upgrade to a bigger house but we need to decide if we should try to keep both existing houses and rent both out, or if we should sell one house and keep one for a rental.

Such a plan could accept a rollover of all or part of existing retirement plan accounts.

Does that exist and am I just not finding it yet?

It sounds like you're missing that so the next option is to simply enforce the existing lease.

I hope to be closing a a rental property next week.The contract signed a few months ago is "subject to existing tenacy."

Based on their lease agreements you can choose to keep the existing tenants or if you plan to raise rents or move in a different direction maybe consult with you're real estate agent that you are working with (who hopefully has experience with rental properties) to see what they say. 

Hey Everyone,My team and I are looking to make an offer on a single family home in Anne Arundel County Maryland just a little outside Baltimore.The current owner has an existing tenant with no formal lease.

It's slightly wider and will not fit in the space currently occupied by the existing mailboxes.To accomplish this, I'd have to pull up the board that all the mailboxes are mounted to and put in a new board that is long enough and then remiunt the mailboxes. 

Another good source of quality tenants can come from referrals of your existing tenants.

If you didn't max out your eligibility up to $453.1K max for most counties in the US, you still may have eligibility left to purchase another (at least partially) home without refinancing your existing.