The lovebirds have set up house together in Corbett's West Hollywood, California, condominium and are now checking out life as a married couple, say friends.
A neighbour says, "They certainly are not hiding their romance. You can often see them laughing and kissing as they walk hand in hand through the lobby of the building. They're friendly and smile at everyone."
According to American tabloid the STAR, the move comes after Derek spotted Corbett's co-star in My Big Fat Greek Wedding, Nia Vardalos, licking his face backstage at an awards show. Derek reportedly gave her lover an ultimatum to choose her or Nia.
Now pals say Derek, 46, and her 41-year-old beau are planning to make it official.
A source says, "John and Bo are really into each other. She moved in with him about a month ago and they're always kissing and cuddling.
"They've been dating for about a year and they are moving things forward by living together in a trial marriage. The next step is to actually tie the knot."