Alot would depend on how extensively it was remodeled to become a 1 family & if you changed it's zoning with the town as well. Also location has a lot to do with it's values for each type of property and how it relates to others around it. A basic reuse of the 2 family as a single but retaining it's real use wouldn't change it's value, taking out the kitchen of 1 unit completely might devalue it though. You'd have to look at who is moving into your area for each type of property and what they're expecting in each property type. If it was an excessively small or awkwardly laid out 2 family it might make more sense to modify it to a good sized or larger single. But like I said, it really is property specific & neighborhood specific, you should consult with several local real estate professionals to get their opinions & make your decisions when you've had several imputs.
Hope that helps,