Washing your hair twice per day should be fine as long as you use moisturizing shampoo.
If you wash your hair twice per day but don't use moisturizing shampoo your hair could dry out because of the loss of the natural oils in your hair that is required to keep your hair healthy and moisturized like it should be.
Washing your hair just a few times a week is recommended unless your hair gets really dirty because of work or something else.
Other than that, there's no need to wash you hair everyday.