S.O.
Knowing the breed of dog would be very helpful---if you have a clue on that.
My suggestion:
keep 2 yr. old away from puppy for a while.
Especially when Puppy is eating or sleeping. You have 2 "babes" and you will have to supervise their time together very closely.
Shelter dogs are usually great dogs..... 2 yr olds are very unpredictable---always be there.