"Splice" sounds like "slice" and "slice" is a word often associated with pizza. "Splice" is a verb which means to join or connect. In film, a splice is when a film has to be taped together because of breakage. This is often seen during the editing process. The joke is about film editor's part time job at a pizza parlor. The word "splice" was used instead of "slice". This joke is playing on the word splice.
Did you hear about the film editor at Hollywood who had a part - time job at a pizza parlor ? He used to ask customers whether they wanted one splice or two .