Company stock listed in multiple exchanges?
It doesn't matter which exchange a share was purchased through (or if it was even purchased on an exchange at all--physical share certificates can be bought and sold outside of any exchange). A share is a share, and any share available for purchase in New York is available to be purchased in London. Buying all of a company's stock is not something that can generally be done through the stock market. The practical way to accomplish buying a company out is to purchase a controlling interest, or enough shares to have enough votes to bind the board to a specific course of action. Then vote to sell all outstanding shares to another company at a particular fixed price per share. Market capitalization is an inaccurate measure of the size of a company in the first place, but if you want to quantify it, you can take the number of outstanding shares (anywhere and everywhere) and multiply them by the price on any of the exchanges that sell it. That will give you the market capitalization in the currency that is used by whatever exchange you chose.