I think Jan Fennell's advice is spot on with cavaliers as well as other dogs -- the existing dog can be very upset to haveanother dog join the household and introductions done on neutral territory tend to go much better than having dog 2 just arrive in the home territory of Dog 1. Better yet if Dog 1 gets several opportunities to meet Dog 2 mefore Dog 2 moves in. If you can go for a few walks with Dog 2 in the next few weeks that would be ideal!
Most trainers I know of suggest walking as an excellent way of letting a new dog get to know the existing dogs on neutral territory. In the case of rescue dogs, I find a group walk is a very good way to ease anxiety and stress. My two are calm and happy dogs and indifferent to having another dog along. For a scared or worried rescue or any new dog, going for a walk away from the home allows everyone to meet and socialise in a friendly way and is more calming for them than walking alone with you. Even very emotionally scarred, abused dogs tend to interact well with other dogs so walks and other forms of canine companionship are one way to help them on to the road toward a new life.
In your case, if the recommendation is to walk them separately until they are both completely trained -- I really think this depends on your existing dog. If he is a source of serious problems on walks the younger dog can certainly learn those bad habits so you'd want to be cautious on how and when they are walked together (maybe you and partner can each take one dog). But the concern of the trainer seems a bit overly wrought to me. Few people are concerned as to whether their dogs walk with military precision as opposed to enjoying a social time out on a walk! I'd certainly not feel you should NOT walk them together at all!
What you WILL need to do is work alone with the pup on walks for training IN ADDITION to group walks, as she will not learn much just following your existing dog. She deserves time and focus herself and needs to learn to respond to you and your cues to her, not your exisiting dog. On the other hand a well trained dog can help a younger new one learn more quickly (especially useful for housetraining!).
One of the reasons many experienced trainers and good breeders do not usually recommend
1) getting two puppies at the same time or
2) getting the second dog before the first is at least 12 months old, 18 months being prefereable is that
1) two young pups will likely bond very closely with each other rather than to the people that own them which is hard to avoid, and can cause behaviour problems as they grow -- and the owners will need to put in a LOT of time with each pup separately to train and so on. Two pups is as hard as twin babies -- everything has to be done twice and unique to each pup andf one pup alone is a serious amount of work and time. Also siblings do not always get along very well as they get older. I know many people who have successfully brought up two puppies together but a good breeder will advise carefully on doing this and it is wise to realise from the start how much work this is going to be. Many good breeders will not home two pups at the same time or two pups from the same litter for these reasons. I don't mean to overly worrry anyone but these are very real worries and extra work will be required as well as a very vigilant eye to prevent problems from the start. Not least because two puppies are so cute playing together than the temtpation can be to just let them spend all their time together and let them get away with whatever they want as it is charming. But what is charming at 4 months is not so charming as 12 months and may need a lot of remedial work. So just be aware, anyone thinking about two pups together.
and 2) if the training of the first dog isn't complete then you will simply not get the time once the second one arrives and you are more likely to end up with two not very well trained dogs. I can speak from experience on the latter point -- I've posted on this before to say that I got Leo when Jaspar was about 9-ish months old and they were a month apart in age. In restrospect I wish I had waited a few more months as once Leo arrived I simply did not have the time nor inclination to work with each separately as I needed to do, as Jaspar was still quite young and only partway trained. I cannot emphasise enough how much more difficult it was than I expected in this regard --- somehow I thought I'd just work both together and also I thought Jaspar was further along than he actually was.
My situation is complicated however by the fact that I live alone and also that I must walk the dogs several times a day as they do not have a garden in which to relieve themselves. So on average we have four walks daily. If I walked them separately -- that would be EIGHT walks or over two hours just walking daily.
In general, if there are two adults -- either you and partner or an older child (eg probably 14 or older) who can each work with each dog/pup separately as well as walks together then there's a much better chance of training success!
For anyone getting two puppies together (I know we have two coming onto the board soon! :wggle: ) just be sure to read carefully and widely on how to handle this successfully.
For example:
http://www.veterinarypartner.com/Content.plx?P=A&A=2043
PS -- I think having two dogs is wonderful for both and one of the best things you can do for the quality of life of your dog. I also think in most cases, two dogs are EASIER than one. Two are a real delight to watch interact as dogs -- and it is far different for them to have a constant canine companion then occasional play dates or walk encounters with other dogs. They are such social creatures that I think they (like cats) benefit from a companion.