I am sure it must be straining to live out of a suitcase and at two different houses.

You should take stock of the capital expenditures that need to be completed.If you bought the property with a BRRRR in mind and have the bandwidth in your budget i would strongly encourage you to focus on the Plumbing and Electrical portions before you get the property locked into tenant rotations.

She is my first ever tenant, and I'm afraid that if bring up the overstaying it will just strain the relationship we have (business - we are not friends)...it is just the two units so we see each other often, and if she leaves or I have to ask her to go I will get someone else in that will be destructive to the unit, and or have the same problem with a SO overstaying their welcome. 

I would ensure that you are comfortable with how they intend to use the property and how many people are going to be staying and whether they intend to rotate the workers or not. 

Your problem could be as simple as rotating the glass handle and tightening it up.

STRs do have liability with the rotation of guests so its important to ensure the liability is removed from you as the owner and contained in an LLC.

Probably have good credit and work history if the company is moving them to US from abroad.This year some said they had to have a 60 day out clause in case the company moved them mid-rotation. 

It almost feels like everyone has be be in a community of hermit crabs...Those who want to move all get out of their shells and move to another shell at once, in rotation.

My current goal for RE is to get some cash flow going so I can be more confident in straining away from my safety net of a job.

I have a few lenders who I rotate through for that very reason.