In theory you could support both sides in an 'agree / disagree' type essay but you would have to make it clear in your introduction when you give your opinion that rather than falling on one side you can see the merits of both.
As I say, it's really difficult to know exactly what you are doing in your essays without seeing some examples.
Personally, if someone wants a higher score, say band 7 and above and they have better writing skills I will advise to discuss both sides just because it will look like a more sophisticated answer and so could potentially make a better impression on the examiner for things such as Task Response.
You can of course 100% agree or disagree with one side and still consider the other side i.e.
In case I totally disagree with the statement, then should I go for both advantages and disadvantages of the development?
Have you got an example of where you have seen a question like that? It would be a bit confusing because if you give reasons why you agree / disagree you are essentially giving advantages and disadvantages so you'd end up repeating yourself.
In proposel essay you have to write your side between two points. Because I write all these argumentative or proposal or opinion essays in one format only.
I introduce my clear opinion in the introduction while my body paragraphs support on both views ..
Yes, I recently saw this real test question in IELTS exam (academic module) held in August 2017 in Qatar.
I found it mind boggling so I post it here just to seek your expert opinion.