2 Properties - 1 Mortgage?

My sister and I have an opportunity to purchase 2 properties in the DC area. They are side by side - semi attached and each property has 2 1br units but are each zoned for 3 units. In your opinion, would it be better for each of us to purchase one separately or should we by them both together under one mortgage?
Please tell me your thoughts.

seperately. easy to transfer or refinance later.