Thank you for the info.
First, i did use VRBO and rented the house being sold for two summers on weekends by the night. When the property was purchased, the house being sold was the only home on the property. The Town let me add another home without surveying and dividing the land. I don't know how that would change your first explanation.
I did think using the one being sold as a residence would save gains. They are next door with a worn path between them, both furnished and stocked with food, TV's etc. I even sleep there many nights. No one could determine which was the residence by looking at the homes or bills they generate. But i guess to be completely honest i'd have to pick the newer home.
So i guess if i am selling half the land with the original home on the property, i would use the original purchase price, determine the value of the land at that time and cut it in half and anything over that sum would be gains. Unless the about five grand in rental income over 17 years changes anything.