
You'd have to train them each separately so they concentrate on you, and then repeat the same training with them together, which takes longer because they're distracting each other. I'd expect the average person (ie me!) to get a pair of pups to a satisfactory standard of training at about 4 years old as opposed to one year old with a single pup.
But then I'm probably a rubbish trainer, because I have other things in my life that need my attention - my job, my family, etc ...