There isn’t really any variable opacity going on with Shutter nor does it perform any summing/subtraction. It’s either letting one signal through or the other but it is very sensitive and can do this on pretty much a per pixel basis with video rate signals into the Switch inputs. You can think of it as a binary on or off between the two channels–there is no in-between state, i.e additive mixing, on a given pixel which Color Chords can definitely do.
If you completely remove the RGB matrix mixer in Color Chords and have the Opacity faders fully up, then I can see some similarity between the two but they’re working on different principles. Basically I wouldn’t buy Shutter if you’re expecting it to be a Color Chords but you should probably get one all the same. 
It’s a really killer compositor for its price and HP.
