[phenixbb] occupancy refinement with constrained groups

Dear all, I am trying to do use constrained groups with occupancy refinement but struggling with syntax. I am trying to define selection1 as 2 different atoms and selection2 as another 2 different atoms. Therefore I want the 2 atoms in selection1 to have the same occupancy and the atoms in selection2 to have the same and the sum of selection 1 and 2 will be 1. If someone could help me with this syntax I would appreciate it! I have tried using brackets to separate selections but phenix doesn't recognize this. I can provide more information if needed, it's just a complicated scenario. Thanks! Nick