Can my students sit Cambridge IGCSE Chemistry 0620 with Cambridge IGCSE Combined Science 0653?

No, not in the same series of exams. In a single series of exams some combination of syllabuses are not allowed and you should refer to section 5 ‘What else you need to know’ in the Chemistry 0620 syllabus to find out the exceptions when combining IGCSE Chemistry 0620 with other Cambridge International syllabuses.