Muhammad - it depends on a variety of factors. The most productive first supervisor / co-supervisor teams that I have worked in have an equally 'shared' role. Often, the first supervisor can be the 'content' specialist and the co-supervisor is the 'methodological' specilialist - both equally important and demanding dissertation development areas - and the two supervisors also 'more loosely' advise and feedback on the other supervisors role as well. Other times - the first supervisor will do the 'lion's share' of the work - and the co-supervisor is more a 'sounding board' for when the first supervisor requires support, confirmation or another perspective. Sometimes, the co-suprvisor is merely a 'sleeping partner' - probably more brought in as a 'recognised' name to attach to the study later on.

Muhammad - it depends on a variety of factors. The most productive first supervisor / co-supervisor teams that I have worked in have an equally 'shared' role. Often, the first supervisor can be the 'content' specialist and the co-supervisor is the 'methodological' specilialist - both equally important and demanding dissertation development areas - and the two supervisors also 'more loosely' advise and feedback on the other supervisors role as well. Other times - the first supervisor will do the 'lion's share' of the work - and the co-supervisor is more a 'sounding board' for when the first supervisor requires support, confirmation or another perspective. Sometimes, the co-suprvisor is merely a 'sleeping partner' - probably more brought in as a 'recognised' name to attach to the study later on.

The co-supervisor is there to give a hand with technical matters from other fields such as statistical techniques, and to act as a back-up when the supervisor goes overseas, becomes sick, leaves the university, retires etc.

Thanks Dr Dean Whitehead & Dr Ian Kennedy for your constructive, helpful comments.

In case it may help, you can check out how the roles of the main supervisor, co-supervisor and mentors are structured at Karolinska Institute.

https://internwebben.ki.se/en/support-doctoral-students

Co-supervisors can have an important 'hands-on' role regarding the methods, analyses and daily research matters

Thanks dear Shireen Sindi for your valuable comments

They main supervisor and co-supervisor are expected to be partners in progress and they should work together for the successful and timely completion of their student's work.However,main supervisor may have to perform the 'leading role'.This is not to say that the co-supervisor is not equally important.

Thanks dear Ebenezer Bowale for your valuable comments

I have not supervised before, so I can only speak from my experienced of being supervised. I really like Dr. Whitehead's answer, except I will add one more thing to content and methodology, which is theoretical knowledge/development. It is the job of the first supervisor to first understand the project and its potential and in what areas s/he can contribute, then to ask another supervisor to contribute in an area where there is a gap in one of the three major areas. So supervisor one may be knowledgeable in content and theoretical, but not methodology, thus the second supervisor may only be able to advise in methods and not the other two areas. Or it can be any combination, however I think it is important that all supervisors are on the same page with what they can and cannot help with as well as this is clear to the student who to go to for help. If all of that is transparent and clear, that should minimize problems with the working relationship.

By my own understanding, the appointment of a co-supervisor is necessitated by the need to supplement the the area of expertise of the main supervisor of a Ph.D. work to adequately cover the scope of the student's work, especially when the work is multidisciplinary. So, a co-supervisor is appointed because his/her contribution is deemed necessary to make the work rich. He/She is, therefore, not like a car's spare tyre but an active participant in the research.

Thanks Dear Samson Olusola Olatunji

It depends on what is excepted of a co-supervisor. He or she may be made a co-supervisor to ensure that student does not suffer in cases where the main supervisor is not available. It can also be in the area of sharing responsibilities. In this case, the student knows whom to consult for each segment as the study progresses base on competence.

But there are some cases like in a split PhD programme where a student is expected to spend one year in a university outside his home institution. In such case, the main supervisor is expected to fulfill the academic requirement of giving necessary guide to the student while the co-supervisor with shorter time may function in the area of sharpening the research experience of the student to meaningfully contribute to the world of works beyond academic exercise.