Odd Language #228: A Singular Use of “Both”

“You are either malicious or incompetent. I grant that both is a possibility, too.”

The issue: “both” usually takes plural verb forms, as in “… both are …”, but malicious and incompetent can be considered two things or just one. (The singular use can be emphasised by adding “you being” just before the “both” or “being the case” after it.)