The two actresses -- who once starred opposite each other on "Grey's Anatomy," before KH left the show in 2010 -- are set to be paired up for Variety's latest season of 'Actors on Actors' ... where stars interview each other and talk shop about their craft/projects.

There were a ton of other duos announced Saturday, but Kat/Ellen's stands out -- as these two haven't publicly sat down and talked for a very long time ... and this would mark the first time they've done so since Katherine exited 'Grey's' under controversial circumstances.

Fans of the show probably remember this time well -- Katherine asked showrunner Shonda Rhimes to be let out of her contract 18 months early ... citing harsh work conditions, which she aired out publicly at the time. Plus, she was blowing up as a movie star back then.

Of course, she was allowed to leave ... and her character, Dr. Izzie Stevens, was written off after 6 seasons. Katherine was a fan fave and a staple in the series, until she wasn't.

Since then, Ellen has walked the line when discussing Katherine's departure -- which she rarely has -- but in 2013 ... she offered a candid review of the saga to the New York Post.

She was quoted as saying, "When Katie left, it was tough. You could understand why she wanted to go -- when you’re offered $12 million a movie and you’re only 26. But Katie’s problem is that she should not have renewed her contract. She re-upped, took a big raise and then tried to get off the show. And then her movie career did not take off."

More recently, however, EP has actually praised Heigl for speaking up the way she did in the 2010s ... consenting that the hours were grueling and not conducive to a healthy workplace.

There doesn't appear to be any beef between the two ladies -- the same can't be said for Kat and ex-'Grey's' costar Isaiah Washington -- but people are undoubtedly interested to tune in to see them talk about the old days. BTW, the interview has already been shot.