I'm not exactly sure what you have done, or what you want to achieve from here.
You can't have the same mobile number on two different SIMs
The phone number is associated with the SIM card, not the phone.
If either or both of the phones are iPhones, turn off iMessage, reboot and turn iMessage back on.
If you want a new giffgaff number, then order a new SIM card (https://giffgaff.com/orders/free-sim
) and activate it on a different account