Yes it's perfectly fine to wash your hair only twice per week.
Washing your hair too often can actually strip your hair of essential oils which can dry out and damage your hair.
I only wash my hair twice per week unless I'm working somewhere and get my hair dirty more often then I'll wash it more often but once or twice per week is good enough for keeping your hair clean and healthy.
If you do wash your hair more than twice per week you really should use a shampoo that contains moisturizer to avoid damaging your hair.