If they break up, what does that do for me and her lease?

Those who give me an estimate, don't give me a break up of costs.

.- $500 - $1,000 flat rate breakup fee, payable only in year 1.This very simple, but higher headline, fee structure better aligns incentives.

Also stay away from couples, what happens when they break up?

Couple two sources of income but they can break up.

I break up each tenat as Unit 1, Unit 2 etc. 

Like a marriage, you certainly want the capacity to change / adjust, but that can lead to a breakup, or a complete change of heart.

You could also add a threshold to break up the two floors. 

Your mindset is the same as "I have a girlfriend and we will never break up because we are in love".

Bought a heat gun from Harbor freight ($10) and wife spent evenings removing multiple layers of stick-on linoleum while I use a hammer to break up a brick wall to make space for a new patio.