Here are 2 things that I can think of:
1) Sharada is Mansukhlal's silent business partner. i.e., she had invested in Mansukhlal's co. When he didn't have any money. Mansukhlal keeps on going to bangalore to meet his business partner. I am sure his sons doesn't know. Reason Mansukhlal doesn't want anyone in his family to know about this is that if they find out about Sharada, that means admitting that women are better than man.
2) Sharada is his doctor and he is suffering some serious illness and doesn't want his family to know about. He goes to Bangalore to get some sort of medical treatment. If he has some doctore in Mumbai, chances are everyone will know Manasukhlal and it would be hard for him to keep secret about his illness. Remember when Mansukhlal prentended to get the heart attack, first think Bhaggu asked him was to see if he liked to inform Sharada about this?
He was doing drama of dying to see who can run this household better than him in case soemthing happens to him.
I don't think Sharada is his second wife. 😉 Mansukh is bad guy but he still loves his family in his own weird way. and also Bhaggu knows who she is so she can't be second wife.
anyway that's my thought... what do you guys think? Feedback welcome....