How should I create a single score with two values as input?
If you're looking for something where A and B are equally represented, consider trying something like Z score normalization (or standard score):

c = (a-u_a)/sigma_a + (b-u_b)/sigma_b


That score equally represents the two, but would be on a smaller scale.  It really shouldn't matter since the numbers are arbitrary, however, if you need to scale it up, you could do something like:

c2 = (sigma_a+sigma_b)*(c) + u_a + u_b