To be able to combine result for admission, you must have passed the subject you failed in one O' level exam in the other. Lets take a student who sat for both WAEC and NECO for example, if the student happen to fail mathematics in WAEC but passed same Mathematics in NECO, the student can easily use the maths he or she passed in NECO to augment for the failed maths in WAEC. Luckily for you also, literally all courses in Nigeria accept combining of O' level results.