Yes, the gastrocnemius is essentially what you see, the larger outer muscle while the soleus sits behind it. See pic below..but you'll also have muscles in the front of the leg, along the shin
You pretty much rule out gastrocnemius assistance when the knee is bent, so seated half raises and toe presses on a leg press with bent knees for example will target the soleus much more effectively than standing calf raises alone.
I also read something interesting today that I'm going to try and that is doing them barefoot and with the big toe 'flexed'/extended upwards as it creates a better stretch on the muscle.