I'm trying to use the CC Burn Film effect in After Effect CC 2015 on two image layers, one on top of the other.
As the burn effect seems random, I can't get it to burn with the same aspect the two layers !
How could I achieve that ?
As with any Adobe program, applying an effect to a layer is only going to affect that layer, not the layers below it.
You'll have to create a pre-comp of the two layers and then apply the effect to the pre-comp.
Select the two layers you want to combine into one composition
Right-click and select Pre-compose...
Your two layers are now combined into one composition
Apply your effect(s) to the Pre-comp
If you wish to edit the individual layers, double-click on the pre-comp and it will open inside the composition window, with your composed layers within.