§ 1.121-2(a)(2) is primarily what I was basing that on, which allows each joint owner to exclude gain "that is attributable to each taxpayer's interest in the property". 

Depending on the state, they may have a tenancy in common and a joint tenancy.  Either way, each person has an "undivided interest" in the property, which means they all have a right to use any part of the property.  But they do still only own a partial interest in the property (a partial ownership percentage).  

So my thought process is that as co-owners who are entitled to 100% use of the property, they pass the would pass the "use test" for the whole property.  But I don't think they would pass the "ownership test" for 2 years for 100% of the property, only their partial interest portion.  

I couldn't find specific tax court cases or other references to this specific situation, so it's possible there is a different way this is supposed to be handled.