Two-headed coins, even pennies, are not considered to be very valuable. This is because the U.S. Mint has never minted two-headed coins on purpose or accidentally.
Most two-headed coins are manufactured by novelty vendors for magicians to use in their magic shows. Other people buy double-headed pennies for jokes or pranks. The machines in the U.S. Mint make it impossible to mint two-headed coins, and such coins that are said to be minted often prove to be false. Novelty vendors utilize a special machine to strike the coins; those found in circulation are minted from this process and are worth very little.