Husband and wife should both fill in nomination forms in names of their spouses and it should be duly recorded by society in its nomination register 

2) in addition will should be executed by both joint owners of flat bequeathing their share in flat 

3) kindly note that nominee is only trustee for legal heirs 

Appointing nominee will be beneficial as on the demise of owner, share certificate transfers in nominee .

Husband can appoint his wife as nominee for his 50% share

Likewise wife can appoint husband as nominee for her 50% share

Thus both can make mutual nomination in favour of each other

The husband and wife both can file nomination for there share

 Either they can appoint each other as nominee for there share or any third person .

If they have joint share automatically after the death of one person the balance shareholder becomes 100 percent. No need for nomination

Yes You can appoint nominees as a husband. 

Since both are joint owners there may not be a necessity to appoint each other as nominee to each other, instead if they will to appoint a nominee then a common son of their's may be appointed as nominee, who in turn may disburse the property to the actual legal heirs of the plaintiff.

1. Both are having equal share, but as rightly pointed out that there should be a nominee for each of the side.
2. And yes, there can be a situation when both will have to become the nominee for each other and there is no bar by law on it.
3. See, even if society says no to it then site that there is no bar by law then how can they deny the same and ask for the reason in detail. And move accordingly as per law.
4.