Yes you can have both an A2 CofC and a GVC at the same time as these grants you different permissions
From 31 December 2020, drone qualifications are going to be based on a risk methodology. The idea is that the higher the risk of the drone flight, the more qualifications you will require.
The system will be primarily based on a 'building block' approach, whereby everyone starts off with doing the Drone and Model Aircraft Registration and Education Service only with the CAA and then if you wish to 'fly closer to people' within the Open Category then you will be required to do an A2 CofC drone course.
If the drone operations you wish to do fall out of the permissions granted in the Open Category with an A2 CofC then you will have to do a GVC Drone Course.
You can hold both an Operational Authorisation (gained by doing a GVC Drone Course and granted by the CAA) and an A2 CofC as these allow you to operate differently based on what aircraft you are flying.