the comma in the node selector syntax means you select the intersection of those two tags, i.e. models that are tagged with tagname_a and with tagname_b. if you want to exclude tagname_a and tagname_b individually then leave a whitespace in the dbt command:
dbt run --exclude tag:tagname_a tag:tagname_b
Note: @Paul P (Target Reply) originally posted this reply in Slack. It might not have transferred perfectly.