Both husband n wife will have to wait for 6 months after mutual divorce. The final proceedings are done in the court. But in Indian society, a husband can marry second time if his first wife does not have any problem.
The second marraige can only be challenged if his first wife does so. Here Aarti n Yash have signed divorce papers but still she will continue be his wife for next 6 months unless court stamps the divorce.
Secondly Aarti has agreed for the second marraige of Yash so will not challenge his marriage so there is legally no problem for his marraige even if he gets married on the next day of him signing the divorce papers.