Completion of two degrees at the same time but admitted at different times.

There is no problem.

You studied M.Sc (Maths) in open university mode during 2015-17. After that you are writing examinations as you have to clear some subjects, I hope. So there is no necessity to go to college for completing the course. Then in the same university, you joined in M.Sc (Computer Science) and completed the course during 2017-19. There is no conflict.

Did you join in the job after completing the above two courses or completed after joining the course? If you have joined after completing the course, which qualification you have shown? Did you show both or only one? Based on these facts you have to decide. Normally there will not be any problem if you show both the qualification.

UGC has very clear stipulation in these matters that though a student is not allowed to do two regular courses at the same time but he is allowed to undertake one regular and one correspondence course at the same duration or with overlap. In your case, the correspondence course was attended by you during the year 2015 to 2019 and you finally completed it in 2019 only. You also did another Masters degree during 2017 to 2019. Your correspondence degree is in Mathematics and regular is in computer science. Which one you had mentioned when you applied for the teaching job and got it? That is important as that only you must write in your records. If you were selected for the present job based on your graduate degree then you show both the Masters degree in your record now. These are two legally valid degrees and there is no problem in mentioning in your records. This is also necessary from another angle that if a better teaching position is available in your school or another school in these subjects that is Maths and Computer Science then you can be benefited by it.

So make it a practice to present both the Masters qualification in your CV wherever you apply and be consistent in your records. Do not give one version here and other version their. That will create doubt and ambiguity. Show both the degrees and be consistent.

In my opinion there is no problem in your case as you have completed M.Sc. Maths from open university during 2015 to 2019. While you got admission in M.Sc Computer science from 2017-2019. So as per a notification issue by UGC . Two degree are allowed only where one course is through regular mode but second one should be in distance or open learning. Two regular courses are nnot allowed at the same time. So there will not be any issue in your case.
If still you have any doubt. I have a suggestion for you that you can also skip one of the master degree which is not much related to your present job.

M.Sc (Maths) from open university and M.Sc (Computer Science) in regular mode. For my teacher job only Under graduation is required.