Double counting can happen if the objective is to arrive at the gross sales at consolidated level.But here the objective is to find out the value of individual units as a separate entity and add it. Here we are using two ratios EV/Sales and P/BV. I think Arun is concerned about the sales figure for the IB unit. Yes you are right when u say that the value addition of the Fin services unit is captured in the Gross sales.
But against this value addition the IB unit must have paid some service charges to FS unit, which will reflect in their its bottom line viz. its value addition. Future prospects of this value addition is reflected in the market price and hence EV. Investopedia says "EV/sales gives investors an idea of how much it costs to buy the company's sales". This ratio is used here because automobile industry is a cyclical industry and it wud be better for an investor to know how much they are paying for a dollar of the company's sales rather than a dollar of its earnings as in P/E ratio. This facilitates comparison with their peers in the industry. These peers might outsource certain components, services or keep some in their umbrella itself. So for a better comparison and hence valuation, Gross Sales gives you the correct picture than net sales. Cant think of a better explanation.
For FS unit, sales is not a concern. Since P/BV ratio is used, i assume it is a listed company so its value addition is reflected both in price and book value.
Note : Eventhough debt is factored in EV/Sales, i feel it doesnt give a good picture coz the value addition is not reflected properly unlike PE ratio.