About three years ago, I upgraded from one sheltie to two... Ever since, I've said I will NEVER go back to having one.
First, the exercise requirement isn't increased at all... Walking two is just as easy as one, and they actually love playing together so both dogs get far more exercise and play then with just one.
It's true that grooming time has doubled, but baths only happen once or twice a month anyway, so a couple of hours extra bathing is totally worth it.
And lastly, I can't overstate the joy that our dogs have received from having more than one furry member of their pack. As pack animals, they REALLY are social, and my shelties have just loved having someone else to play with... When we lost one of our dogs last summer, the other was SOOOOOO sad, he only lasted a couple of months before passing away himself--and we truly believe it was because of a broken heart. He missed his brother soooo much. I'd never seen an animal grieve like that. Knowing how much they loved one another, I personally would never want to deprive them of having that bond with a fellow canine family member.
That being said, all of the above is my opinion only, and I fully understand that not everyone can have more than one dog, so I hope it doesn't sound like I'm suggesting that it can't be done, otherwise.
Best of luck as you figure out what works best for you and yours!
DKBA