Hi there, @sikirk
If the phone takes a single SIM then, No sorry, You can only have one number per SIM.
However, if your phone is dual SIM phone then yes you can place a second SIM. if you wish to activate a second SIM, then you need to open a new account , along with a new username, all other details can be the same as your first account, such as password, email, and payment details.
Hi there @sikirk
You can if you own a dual-SIM phone. If not, you can't. Please note that it is not possible to manage multiple Giffgaff SIM cards from one Giffgaff account. If you activate a new Giffgaff SIM, a new additional Giffgaff account must be created with different credentials I am afraid.
If you wish to order and activate more Giffgaff SIM cards, each SIM card must have it’s own separate Giffgaff account. This means using a different Member Name and preferably a different Password. You can however use the same payment details (credit/debit card or PayPal) and email address for the new account, should you wish.