Can my students sit Cambridge O Level Chemistry 5070 with Cambridge O Level Combined Science 5129?

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 5070 syllabus to find out the exceptions when combining O Level Chemistry 5070 with other Cambridge International syllabuses.