It is only possible to have one voucher active on your account at any one time. The addition of new vouchers will overwrite any unused vouchers, or ongoing discount vouchers that are already on your account.
While it's not possible to use two vouchers at the same time, when making a new purchase, you can add your friend referral credit reward to your account first via the ‘Redeem A Voucher’ section in the drop down menu, which will then automatically be used to go towards your next purchase.