The Reason for using the Acidophilus plus Bifidus, may be that, then, Depending upon what you choose to eat for your meal, that then "ALL" of the needed probiotics have been supplied, and the one which is needed will multiply and do it's job, while those not needed for that food will not grow, but may go Dormant til needed !
As each kind of food may require Different Bacteria and Enzymes to be Digested properly !