^a the subject is plural but the OBJECT is singular so it is not like God is telling us they sharpened the tongueS (pl) in their mouths... this singular noun lets us know it is their language they have "sharpened"

^b ακοναω: ακονησαν3rd person plur imperfect in act sharpen whet [credit Morphology provided by Morpheus from the Perseus Digital Library at Tufts University and An Intermediate Greek-English Lexicon by Liddell Scott)