Umm... I thought it was obvious, but apparently not.
Christina & Dustin Draper
were married.
Christina and Dustin split/divorced.
Christina & John Buhs are now together, leaving Dustin alone.
There are (2) separate/independent guys here, they are not the same person.
I would place my odds on Dustin simply using Christina and John's names as a hefty "screw you both". Exs often do this to get back at the other. It would be easy enough to heist OR take a picture of his ID card during a visitation to see his kids at Christina's place. If he took a picture during a visit, John would be none the wiser... until now.