You can't have two sims on one account matey one sim only so you will need to perform a sim swap matey here's how to do it
NOTE= Sim swaps will only happen between 04:30am and 9:30pm_
1. Go to https://giffgaff.com/activate
and enter the 6 digit activation code from the back of your sim pack.
2. The next page willl display information about your old and new sim click where it says yes I want to swap my sim then click the confirmation button
3. Put the new sim in your phone and restart your phone it should activate straight away. If not restart your phone after about 30 minutes.
P.s if you need anything further just post back