We brought Monty home as an 8 week old Pup when Belle was exactly 12 mths old. Belle was a good Pup a bit of chewing , a bit of puppy biting but that was it really , a pleasure to own. Belle did have her nose put out she became a little quiet and more clingy than usual if thats at all possible
But she was fine with Monty, Monty too was a really good pup, no real puppy biting but liked a good chew, only really stopped a few mths ago ( he is know 18 mths) They got a long famously after about a week and now live together in harmony, Belle is the boss and Monty never forgets it
BUT I have been incredibly lucky and its not usually as straight forward. As for training I never trained them seperately Monty picked it all up from Belle when she was doing what she was told she got treats , Monty did as he was told so he got treats. But I know a lot get trouble trying to train no 2 . But there are downsides, I find taking 2 out to places (village fairs etc) hard work whereas one is a pleasure, so we don't tend to take them so much on social occasions whereas we would if we just had one. There is twice the expense, double insurance, food bills and grooming costs, you also have to groom 2 dogs which can be time consuming and instead of one doing the wall of death you have 2
I love having 2 dogs and I hope they enjoy having each other too but I am well aware that I have been vey lucky and there are lots who havent been so fortunate . We have had a fair few COLers having to rehome the second dog as despite their very best efforts things just couldn't work out.
IMO 12 mths was a good time to introduce the 2nd pup as Belle was not too set in her ways and was very happy to play with a boistrous pup, but she had enough maturity to sort him out when he stepped over the line. Sorry for the long post , enjoy your pup for now, plenty of time yet before you will be ready for another