(follow up) Thanks Mario - in this case, the parking space is not shared, both parts of the tandem space belong to the same unit. So, the question is how much incremental value the tandem provides as compared to a single space. The space is in a garage.

The Visa answer... priceless, assuming you have to share the tandem space with another condo/TIC owner in the building. This can cause severe headaches when your garage-mate is not home and you are stuck behind his car because he decided to take Muni that day. I would put the value at approximately $25,000 for a shared tandem space and upwards of $50,000 for a single space if it is a true garage. Regardless of price, strive for the long-term happiness.