Advocacy for a co-flat owner

X can very well appear on behalf of his client and if the society hires the serevices of advocate X, then there is no infirmity in X appearing on behalf of the society

Yes.  Why not?  X can take up the case of Y.

X, an advocate and owner cum  member of the Co-op. Hsg. Society can represent Y, owner cum member of the same Co-op. Hsg. Society. 

But in no way X, the advocate by profession and member cum owner of a flat in the same Co-op. Hsg. Society can represent the society alias the Managing Committee. 

Hope things are clear to you. 

Very soon the Coop. Housing too is also coming under the Union Minister of of Co-operstion.  State will have less power and so the Managing Committees. The terminologylies like "Managing Committee" or "'Board of Directors". It will be called then "RESIDENTS' WELFARE ASSOCIATION" only.