The setting is an Earth-like desert. A brother and sister find an astronaut they believe to be an alien, although he's entirely human in appearance (as are they). I don't remember much about the plot, except that the astronaut believes he's an Earth human until he notices the misspelling of "Tucson" as "Two Sun". He then looks up into the sky and sees two suns, finally realizing he's not on Earth.
I've googled, asked Reddit, trawled through IMDb, and searched this stack, but can't find anything even resembling it. It may have been a made-for-TV movie or a straight-to-video movie, and seemed to be aimed at kids/preteens. I don't recall any well-known actors from it.
Probably from the late '80s, early to mid '90s.
I think the astronaut was on a normal mission, thought he crash-landed back on Earth, but was really on the planet with two suns. I can't recall if/how the alternate Earth was explained. The kids, I think, are trying to help the astronaut figure out where he came from. There may have been government involvement-type suspense.
I checked Wiki's list of science fiction films from both the '80s and '90s and got nowhere. I swear I'm not imagining seeing this movie. Also checked all episodes of The Outer Limits, The Twilight Zone, MST3K, and Ray Bradbury Theater...still no hits.