Yes you can have both.
To choose the way,that you would like to top up,there are a few options.
1) Top up using a giffgaff or O2 voucher,the minimum top up is £10.
Simply enter the voucher code,either by dialing 43430 from your handset or go online.
2) You can also top up your credit by using your credit/debit card.
To top up your airtime credit,click onto,or visit the link below,cheers 😀 .https://giffgaff.com/mobile/buy