Well, actually there is a word. It's "Quincena":) (/keen-zen-nah/)
Literally, it means 15 days:) Buuuut, it is used to refer to a period of two weeks:)
For example, "pago quincenal" is a payment you get every 2 weeks:)
But of course, you can just use "2 semanas" like Marcos said:)