监考 is a verb-object compound (离合词) meaning 'to proctor an exam'. As a separable verb, the two characters split when measure words, durations, or aspect markers like 了 or 过 are inserted between them — e.g. 监了两场考 ('proctored two exams'). You cannot put an object directly after 监考; instead, the 'exam' part (考) already carries the object meaning. The verb part can also be reduplicated (监一监考 'do some proctoring'), and the compound is often used attributively in 监考老师 ('invigilating teacher').