To topup another phone using a voucher, dial 0844 879 4180. You will be asked for the giffgaff phone number you want to top-up and your top-up voucher code.
Is the SIM card already activated? You can top up another giffgaff sim from your own account/phone By debit card here Http://giffgaff.com/topup/altruistic
Or by voucher calling #31#43430
Or do you mean you want to activate/top up the other sim?