I will be using the Airport Express from airport to Kowloon station with my wife.

According to your situation, you are suggested to buy Airport Express Travel Pass. It contains one or two Airport Express single journey(s) plus 3 consecutive days of unlimited travel on MTR (except Airport Express, East Rail Line First Class, Lo Wu and Lok Ma Chau stations). After three days, you can recharge some money on the card and enjoy some discounts when taking public transportation. When you leaving HK, you can get the refund back. Don't worry.

The discount 'Two Travelling Together' you mentioned is only valid for airport express. You still need to pay money for your five days transportation in this area. So in general, Airport Express Travel Pass is the most suitable one for you.