EXOD691 "You shall not impose interest upon him" [this verse]. This is an injunction directed towards the witnesses [i.e., those who witness a loan where interest is charged. The injunction applies to the lender and guarantor, as well as; see Bava Metzia 75b]. As such, the pasuk says "You shall not impose," in the plural, subsequent to saying, "You shall not act towards him as a creditor" [this is written in the singular as it refers only to the lender] since it is an injunction that is directed towards the witnesses [i.e., it is plural because it applies to the two witnesses].