AQA have arranged their combined science (i.e. two GCSE science exams) in two different ways. They call one 'synergy' and one 'trilogy'.
In trilogy it is arranged, and examined, under the heading biology, chemistry and physics.
In synergy it is arranged and examined more under topics (e.g. life and environmental science or physical science.)
I think they pretty much cover the same material, and both count as 2 GCSEs, it's just the way the material is arranged. It's to help schools with different teaching arrangements really