What it does
I'm not aware of "Two-Headed Coin" as an official Magic: The Gathering keyword or ability. It may be a misremembered name, a custom/joke card, or confused with something else.
A couple of real things it might point toward: Two-Headed Giant is a multiplayer format (and sometimes appears in card text) where teammates share a life total and turn. Coin-flipping cards (like Krark's Thumb, which lets you flip two coins and ignore one) are a real mechanic, and Krark, the Thumbless is a popular Commander built around flipping coins. There's no official keyword combining the two.
If you saw this on a specific card or in a particular context, double-check the exact wording in the Oracle text via Gatherer or Scryfall—I'd be glad to explain the real ability once we pin down the correct name.
Iconic cards with Two-Headed Coin
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