Two sittings might be acceptable, but note the following:
1. They will usually check that your names are exactly the same on each certificate. If it isn’t e.g., because of marriage, then you will need to also submit your marriage certificate/affidavit as evidence of name change.
2. Also, the universities that accept two WAEC/NECO certificates will most likely expect both qualifications to have been awarded in the same year or the year after. If both dates of the certificates are too far apart e.g., 2011 and 2021, the oldest one may be rejected.Nonetheless, you should check with the university you are applying to whether they will accept two certificates from different sittings and under what conditions they will be accepted.