i think none of the founders married each other. Reasons-
-we know for sure that after salzar left hogwarts he got married and had children because his bloodline was continued until voldemort. He had said that the chamber would be opened only when his true heir comes back to school so it couldn't be that Helena was his child. also. i don't think he was the kind of person who'd leave his family behind so he couldn't have married Helga either.
- Rowena couldn't have married godric because helena was nothing like godric. she was too scared to come back and visit her mother even when she was on her deathbed. also. if godric was her father he would have made sure that she returned back to them.
- as for helga and godric, well, it could be possible that they were married but i dont think they were. why? dunno.. just a feeling.
so, i think that all four of them might have had crushes on each other but nothing more than that. would love to know what others think of this theory!