Different Strokes: The Story of Jack & Jill... and Jill
One of the final films in which Dana Plato acted before committing suicide by drug overdose in 1999. The theme is that Jill Martin (Plato) arrives in Los Angeles to supervise a photo shoot with Jack. Jack is involved with a woman named Jill (not Jill Martin, just "Jill") and the two are experiencing some difficulties in their relationship. Jill and Jill Martin then come to terms with their repressed lesbianism, and wacky mayhem ensues.
The title is an obvious play on Diff'rent Strokes. The movie went direct to video in 1997, but gained some exposure (no pun intended) in the E! True Hollywood Story on Dana Plato which aired in 2000. It is available on VHS and DVD wherever videos are sold.
No, I haven't seen it. Thanks to IMDb and E! for filling me in on the details.