I have heard a lot of horror stories from people who have bought 2 family homes and then tried to sell one of the units as a condo. The horror stories usually involve the potential buyers feeling like the condo docs make them second class citizens as definitions of common areas, parking spaces, etc... tend to favor the person who had the document drafted Also, it is usually the case the the original owner keeps the top floor and sells the bottom. The potential buyer realized that this can be a no win situation if they have problems with overhead noise as they only have a 50% vote and therefore no power. What ends up happening is that the potential buyer complains that the condo docs treat them as a tenant rather than an owner. If you are going to do this, make sure your lawyer structures the condo docs in a way that they would be approved by a potential buyers lender.
That said, many of these 2 family condo conversion sell every year. I am just giving you a few things to consider when having the documents drafted.
Brecht... they already own the home, I think condo'ing it could relieve a lot of potential economic pressure in these crazy times. What if they can cover their current mortgage with the sale of the other condo? We don't know when they bought the property... I totally agree with your estimates though.