2 for 1 vouchers and discount coupons can generally only be redeemed on the day of your visit when purchasing tickets on the door.
For a 2 for 1 voucher you will need to purchase a full priced adult ticket to redeem the second ticket for free. For any discount coupons please check how many people the coupon allows the discount for as some are just for one single admission.
Please be aware that most vouchers are required to be printed out for us to redeem at the admissions till.