Few rules you will need to follow to make sure you are doing a real a/b test 

* Campaign should not be edited or changed for at least a month once we started 

* Both campaigns should have the exact same budget

* Screenshot clients campaign stats before you start to have a benchmark

1. Copy existing campaign

2. Add your agency name in the campaign name

3. Remove all your existing audiences from the campaign copy and Choose Include Audiences / lookalikes:

Cold

Warm

Hot

4. Set the correct budget