Before or after exercise.
Combining the two provides power and endurance, and their attributes complement one another: creatine gives you strength and reserves, and BCAAs give you muscle energy.
You should take creatine before a workout (some 30 minutes before), and BCAAs can be taken during or after a workout.
There’s only been one small study on when they should be taken, so it’s uncertain whether it makes a difference whether you take BCAAs before or after exercise – so go for whichever suits you and see how it feels.
