I have done this twice now, the first time back in 77 with my Beagle's pup with my friend's Lab, Blackie, and Joe, my yellow Lab. They never really had any problems and to me, it was much easier than a single pup. Joe was a huge "daddy's boy" who was about as cuddly as I thought was possible. I thought wrong. Blackie was an odd dog, she probably had mild seizures her entire life, but we just thought she was spacy. She really was only happy when she was catching, and eating, birds. She wasn't a "player" and had no use for toys.
My present dogs, King and Molly are littermates. House breaking was so easy, it still boggles my mind. Molly was practically instantanious, and King took about a week to get it 95%, and another week to get it down totally. My mother lives with me, and Molly is "her dog", mostly due to the amount of snacks Molly gets from her, I think. Molly eats continuously, an amazing amount, and never gains any weight. King is so into me it's insane. He follows me around everywhere, and if I go outside without him, he will moan and groan and eventually, howl. The whole "bonding with each other" stuff is nonsense, in my opinion.
My cousin recently had to put his Golden Ret down due to cancer, and he decided to get two pups and try two at once, and is totally convinced, "It's the way to do it". He was afraid it would be twice the work, it's about half. The only negative is the expense of two pups, something he can easily handle. They tire each other out, and sleep better, and it seems to help with chewing too, though it's impossible to know how much into chewing they would be anyway.
When it's time for me to get pups, hopefully a long time from now, it's going to be two for sure.