This is really a 2 part question, so please forgive me if it's not posted in the correct topic area, but I figured the best place to post it was in the topic area that needs to be addressed first, for that's the area that is holding up my 2nd question. 

So, the background:  I have a "leg" that I'm modeling, which is to be split into 2 pieces, which will then be joined together to form one cohesive leg.  The 2 pieces will be joined in a fashion similar to a door hinge application - one side will have "male" protrusions, which will fit into "female" depressions, while the other side will have the same features, just alternated.  The leg has a hole through the center where a pin will be placed, running down the hole & through each protrusion, thus holding each side together to the other side. 

I modeled it as 1 part, but with 2 configurations (left side, and right side), as can be seen in the attached file:  "Barbed_Leg_Configurations.jpg"  Each "side" has certain applicable protrusions either suppressed, or unsuppressed, as needed.  The file shows the 2 legs, and the arrows show how & where the 2 pieces are to fit each other.

The 1st question is:  How can I model the depressions?  I know there's a way to take the location, size, and dimensions of the protrusions & use those parameters to create the depressions, but I haven't had the need to perform this type of modeling as of yet, so I'm not sure how it should be accomplished.  I also know there's a way to do this such that if I decide to change the dimensions of the protrusions, the depressions should update their dimensions accordingly, such that they are always "relative" to the dimensions of the protrusions. 

I have used equations before to control relationships of dimensions - is this how it would be done, or is there a different way within the actual design (something like external references, if that's the correct term)? 

Then, after I get this done, I'd like to ask my 2nd question: 

Is it wise to just continue modeling this as one part with 2 configurations, or should the left leg be 1 part, and the right leg be a completely new (different) part? 

The reason I'm asking is - once both pieces are modeled, I will select them to go into an assembly, and I will mate them coincident to each other's front faces such that the protrusions on the left leg will match up w/ the depressions on the right leg, and vice versa. 

If I haven't explained something clearly enough, please let me know.

Thanks,
Rob