What is the value of a double headed penny dated 1964 on one side and 1974 on the other?

This is a magician's or novelty coin, manufactured from two real pennies. While it's theoretically possible to have a double-headed coin come from the mint (though so unlikely that you may as well say it's impossible), there's no way, given the US Mint's practices, to have two sides show such a 10-year difference like that. (The same is true of the British Royal Mint, in case you're referring to a true penny, rather than a US cent.) These sell on eBay for a few dollars.