Originally posted by aly
I think its best to introduce the puppies to the cats while the puppies are still small. The cats will be more likely to see them as threats if introduced when they are bigger. It will be less stressful on the cats to meet them smaller. The meetings should all be supervised initially.

I strongly discourage getting 2 puppies at once, especially littermates. If your "friend" wants 2 dogs, I would get one puppy, then wait a few months and get another. Siblings can have rivalry problems since they are the same age and size. Its even worse when they are also the same sex. They also can have overbonding issues and that can cause problems for the humans. When the dogs overbond, the humans are not interesting at all and the dogs just won't listen to them. If you start with one puppy and add another later, you could be sure that you'll have dogs of different ages, sizes, and sexes. This will GREATLY reduce your risk of behavioral problems. Also, especially if you're a first time dog owner, trying to train and housebreak and care for two puppies at once is a LOT of work. I'm really experienced with dogs and my two foster puppies are driving me wacky. When my friend was babysitting one and I had the other, life was much easier. Of course everything boils down to the individual personalities of the dogs, but getting 2 littermates at the same time is usually asking for trouble.
I'm reprinting Aly's advice because it bears repeating. A long, long time ago, before I had met my husband, he adopted two female littermate puppies and experienced all the problems Aly was describing. Any of us who have raised even one puppy at a time know what an exhausting job it is to socialize, house train, and just generally get a puppy to develop the skills needed to be a good companion animal. Throw two puppies in with a house full of cats and I can foresee the possibility of the towel being thrown in with one or both.