It would probably make more sense to do this as a bar chart. Kibana pie charts let you pick a way to aggregate the data and the metric to show for the aggregations. You are looking at all the data as one aggregate and want to get the sum of 2 different fields in the data.
In a vertical bar chart, you can have multiple horizontal axes. You can make a bar chart where one axis is the sum of
count_A and another axis is the sum of
count_B. Here is a screenshot of how to set that up in Lens:
You can leave the "Horizontal axis" option blank to view all the data as one aggregate. Then select 2 different vertical axes as "sum" of the fields you want to see.