The Mystic Circuits Ana should do the job, but I want to see if I can solve it with my existing modules. I think what I can do is use Maths subtraction, and then invert negative results. So:
CV1 = 2V, CV2= 1V, Subtraction = 1V
CV1 = 3V, CV2 = 5V, Subtraction = -2V, invert that to get my 2V
FYI, I want to use this with a Theremin. I don't play well enough to play in tune with it but I do not want to just quantize the output either, may as well play keyboard then. So the idea is split the CV out, quantize 1 side of the split and compare that to the unquantized signal. The difference is then used to drive something else. Could be a modulation depth of sorts: hit exactly the right CV on the Theremin, you get the 'pure' note/tone. The more you drift away from the correct pitch the greater the voltage difference, the greater the modulation depth. Until you are far enough away from the quantized pitch to hit another note and then the unquantized voltage is compared to that again.
This will lead to some odd results as most scales will not have equidistant 'gaps' in CVs but the effect could be nice. And hide that I cannot play Theremin to save my life.
I'm a bit surprised this 'get the difference' function is not more common, I think you can use it for various interesting purposes.