Hi,
Loose stools or diarrhea can be caused by many things. Is your puppy happy and healthy otherwise? Running around with the other pup, or just wanting to sleep? Still interested in food?
Common causes of diarrhea that should be ruled out are stress, change in diet, overeating, or intestinal worms.
How long have you had the puppies? They both may be stressed due to the change in environment and if you haven't had them for too long I wouldn't be surprised if the diet change is causing problems.
Puppies should be wormed (tablets are available from your vet) every 3 weeks until they are 12 weeks old, and then every 3 months until they are 1 yrs old. This is a simple inexpensive thing to do to rule out worms as the cause of the loose stools.
If your puppy is showing any other signs of illness (vomiting, unwilling to eat, not wanting to play etc) I would get them checked out by a vet. Young animals can become very sick very fast, so if you are at all concerned, it is better that you get them checked sooner rather than later.
Hope this helps you. Let me know how it goes
Kate