The Carpenters as you know were the Adult Contemporary staple of the first half of the '70s, but in the middle of 1975, right when the Carpenters big hits stopped, along comes The Captain and Tennille, and like the Carpenters, they had this "soft rock, bedroom pop" feel with the song Love Will Keep us Together, and they sometimes would cover old Motown hits (Shop Around as opposed to Please Mr. Postman), and pretty much lasted until the end of the decade. I feel Captain and Tennille picked right up where the Carpenters left off. They both are related male-female duos (Caprenters were siblings, Captain and Tennille were husband and wife). I even found an article from 1977 discussing their similarities. Plus, they both recorded for the same label (A&M).

An older topic from a couple years ago discusses a comparison between the two groups here:http://www.inthe00s.com/archive/inthe70s/smf/1194157326.shtml

I didn't find them that similar.  IMO Carpenters >>>> Captain & Tennille.  Nobody could hold a candle to Karen Carpenter.

It's likely that the two groups appealed to a lot of the same people, though.

As above, the same audience may have liked them, but I can't find any similarity in their music.

That having been said, Toni Tenille did not really get credit for her vocal skills in the same way that Karen Carpenter did.  That may have something to do with that silly variety show.