Yes it can be used for admission in some universities and colleges. For example, if u didn't make a good grade in English in WAEC and you made it in NECO, it can be used. Note: it is not ideal to combine different results it is better to have good grades in one of them.
Yes, I used it in IMSU. In my WAEC result, I had d7 in English while my NECO is C. before I used to think that it can't be combined together, I went and waste money taking GCE which the whole result was bad. Note: Just make sure the two result is of the same year.
Of course, they can be combined. As long as the subject Combination is good. You may not have passed mathematics in WAEC but passed it in NECO, you're good to go most especially for Federal and State Universities