Please start by uploading a Tableau Packaged Workbook including your sample data so we can use that as a reference.
I'm not certain I fully understand your need but here is a thought assuming that your A and B are separate population measures
- Create a ratio of A to B so that you now have a single measure
- drag your new Ratio to to color tile on you map viz
- open the color tile and select Edit Color -
- you can select the divergent color scheme you want
- either use full range of color or vary in steps to get the intensity you want
Does that help
It's all proprietary data, so it will take me a bit of time to build a mock example.