Most things go out the window when theres another dog for your pup to play with...if your referring to training this is best done one to one as you could undo all the good work you have done with another dog being there.
True rearing pups and training them is best taking 1 a time ,because if you've done all your retreving n stuff then take both out together it seems as if everthing you've done way them seems to go out the window,,,,n turns into a tug off war contest n you dont want that,,good luck.....TRUE statment undisputed..