Do Dooni Chaar is something we learn even before we go to school. It's the first thing that comes in the table of two, actually second to be precise. Do Dooni Chaar exactly means Two times two (is) four.
As for the Rishi Kapoor-Neetu Singh starrer movie Do Dooni Chaar, it's a story of a middle class family whose needs double as the two children grow up to be adults. Hence the title, Do Dooni Chaar, or two times two is four.