Double Entry Method of accounting is taught to Accounting students; the sick-minded amongst which realize an opportunity to help coeds with their homework.
The innuendo is double penetration of multiple, usually female, orifices by an erect male penis (AKA throbbing hot cock).
Joe said to Sally, "Let me come over and help you with your homework. I am an expert in the double entry method." Sally learned what half-n-half was, but still flunked accounting.