You need to do a lot more than "put a variable", whatever that means.
Firstly, hangman is a one player game by definition. How exactly do you want it to be two player?
Will there be two men to hang? Or do you want alternate players to enter letters? Is there a winner or is it a collaborative game? If a winner, how will that winner be determined?
Then once you've determined how it's going to work, you'll need to add code to implement those changes. If it's as simple as two players working together to hang one man with no overall winner then you'll probably need to do little more than alternate between
cout << "\n" << "->Player 1 Enter a letter: ";
cout << "\n" << "->Player 2 Enter a letter: ";