I get your point. Sorry for misrepresentation. After reviewing it again, I get that both are being minimized. Right now my top priority constraint is minimizing total miles traveled and secondary constraint is to minimize the total no. of drivers used in case there are multiple solutions for minimum miles traveled.
The formula you mentioned is what I have in mind to implement for the weighted sum i.e. w*f_1/f_1^* + (1 - w)*f_2/f_2^* where f_1 and f_2 are the objectives, f_1^* and f_2^* their best known values (from the first two runs), and 0 < w < 1 the weight to combine them.
But if I take f_1^* as the best know value from my previous runs (which will give the minimum distance to travel), the ratio of f_1/f_1^* could be more than 1. My understanding was that the normalized scores should be between 0 and 1. Please correct me if I am wrong.