I'm not sure I understand what you want to achieve here but it might be interesting to use tables calculation : Table Calculation Types - Tableau scroll down to Percent of Total calculation. And then, when you create you double axis, you have to compute these 2 % of total calculations on the Date field. So in the screenshot below, the total of your bars is 100% of sales and the total of the line is 100% of profit.
Is that what you were looking for ?
Have a great day !