Oh yes!
Two are better than one.......but four is better than three!
I would never have a single dog again. I say that because the change in Sapphi when Max arrived was immense. I would have said she was a happy girl before his arrival but she just blossomed when he arrived. Sapphi was 21 months old when I got Max, I wouldn't have got him sooner as Sapphi was a bit of a challenge as a pup/adolescent and by the time we got Max she'd grown up considerably. Interetingly she completed that growing up literally overnight of Max coming home.
I went for a boy because I'd been told a fair bit that two bitches would be asking for trouble and that they'd fight all the time. Well boy and girl worked out very well, couldn't have been better, however a few years on and rather more experienced I know so many people who have two, three and four bitches together without any problem.
We decided to add to our family when Max was 11 months old. This decision was based on Max. He arrived at 8 weeks old a grown up, he has been a dream to own he's never exhibited troublesome puppy behaviour, there was even no sign of adolescence! Barney came along when Max was 11 months old, Sapphi and Max were a little confused by what this tiny "thing" was that just slept at my feet all the time. A few days in Max and Barney were firm friends and within the week Sapphi had accepted his presence. Within two or three weeks they had all gelled together and were having a whale of a time together.
Max didn't change one bit because of getting Barney when we did. Barney was our first cocker and within the first couple of months he'd thrown up things to deal with the others had never thrown up but I think because I picked up on them and dealt with them immediately we didn't get problems later on. From being around six months old he's been so good and a grown up little man, again we've not had any signs of adolescence.
Morgan arrived when Barney was 11 months old, once again his arrival didn't lead to any problems. Barney hated him for all of four hours!
From then on they have been inseparable.
I really think that it depends on your dogs AND your circumstances as to how many dogs you should have and when you should introduce them. I knew with Sapphi there was no way I could have added in a dog earlier, but it was a different story with the boys. However, I am at home all day, I don't have kids, the dogs are my life and my life is run around their needs. I spend quite alot of my time training them, and training them individually. I attend training sessions with each of them too. I believe that my circumstances has played a huge part in how my boys have grown up quickly and I certainly do not think that everyone should do as I have done. But I do think that deciding when the time is right to add another dog you should base that decision on your dog, your circumstances and the time you are able to dedicate to them.
I haven't had any problems at all with having three boys together and I can hand on heart say that my four have never even so much as have had a squabble. I'm not planning on adding any more to our happy band for about 4 years, but when I do it will be a boy.