Raycast performance - 2 layers + 1 or 2 raycasts ?

I just want to know something about raycast performance.

Lets assume i have 2 layers: enemies and ground. Im performing raycast in update to have continous RaycastHit data whats mouse is pointing at. Sometimes i need to use RaycastHit data for ground, sometimes for both ground and enemies.

So now i need raycast over 2 layers and i want to know if performing one raycast for ground, and second raycast for enemies would be much worse in performance then one raycast for bitmask containing both layers.