What is a good count or spot to throw your 2-seam fastball?
My advice is to throw it all the time if it has good movement! Throw the 4 seamer once in a while in particular circumstances. If a batter is late on your 2 seamer, or you want to high inside with some extra umph. Other than that, throw your 2 seamer to try and make the hitter miss the sweet spot of the bat.
Assuming it sinks, double play situations seem like a good time to throw it.
My son is a lefty and has real good movement on his 2-seamer. He will throw it probably 60% of the time if not more. Why throw a flat pitch when you can throw one with a lot of movement? Son throws his about as fast as his 4 seamer, little to no velocity difference.
As a general rule, I ALWAYS used to throw 2 seamers to the right side of the plate and 4 seamers to the left side of the plate. Didn’t matter whether I was facing a RHH or LHH.