Yes, you can buy the tickets with half fare in advance without the possession of a Half Fare Card. You just need to have both at the time of boarding the train and when your ticket is checked. But beware that half fare tickets are invalid without a Half Fare Card. Also, buying in advance is only useful, when buying Saver Tickets and Saver Day Passes. Regular Point-to-Point Tickets cost the same, whether you buy in advance or just before boarding.