By your business you mean airbnb, correct? I assume so.
AirBnb is the end game here so I would just get cooking with that. It seems to me that you know what makes the most business sense but struggle with figuring out how to navigate the human aspects.
Ideally you would test the business idea with just one unit and keep the benefit of having that MTM income from the other unit. If the business works, you will scale. If it doesnt work, you will go into year leases. Thats the ideal situation from a business standpoint, correct?
You bought this property for business so I think it stands to reason to first commit to the solution that makes the most business sense. Once you are committed to that, we can figure out how to delicately get over the awkwardness factor. We wouldnt want mere awkwardness get in the way of making good choices, right?
It appears that there are two points of awkwardness. Tell me if Im right.
1. Which tenant to move out and how to justify just moving one out and not the other one?
2. How to communicate with the staying tenant about their future?
If I am correct about the points of awkwardness then here is how I would answer the first question. Is there business reason to move one tenant out before the other? Is one of the units better suitable for airbnb? I would say an end unit would be better than a middle unit. A bigger unit would be better than a smaller one. Make it a business decision.
What words to use with the tenant? I appreciate that you have taken good care of your unit. When I bought this property I did so with the idea of using it as a vacation rental. So starting next month I will need your unit vacant.
What about the staying tenant? I feel like it would be courteous to tell them that you are currently experimenting with the other unit as a vacation rental and that there is a possibility that you might need their unit vacant in the future too but for the time being they are more than welcome to stay and you heard good things about them from the previous landlord (if that is true).
Thats my two cents. Let me know if I was incorrect in my assumptions.