Welcome to the board!
Most people (and breeders and trainers) would really recommend waiting until your first dog is at least 18 months and fully trained -- housetrained and obedience trained -- before adding a new dog or puppy. A new puppy will completely dominate your time and schedule and if you haven;t fully given time to train your first dog, and if they aren't fully housetrained, training the first will usually just end `t that point and the new dog will not benefit from the self control of the first dog. In essence, you will have two puppies at the same time, not generally recommended.
Much of the advice offered here will apply:
http://board.cavaliertalk.com/showthread.php?t=10549
A;so this is excellent advice:
http://www.diamondsintheruff.com/secondpup.html
Also this is very good advice as there are really many things to think about (although the pack issues are not that big a problem generally with cavaliers, especially not just two of opposite sex):
Timing
Getting two dogs at once is a popular idea, but seldom a good one. It takes a dog more time than you would think to become fully integrated into the family. If you bring in another dog before that time, some things can be lost, including best bonding between the dogs and human beings. A safe interval from this point of view is to wait at least two years between bringing a new dog into your family.
This does not apply to professionals or serious hobbyists whose dogs do not actually live together. With strategic separations, the dogs don’t form a pack. This allows dogs to live at one location in combinations and numbers that would be too stressful if they had to slug it out for pack order.
You’ll also want to think about what it’s like to deal with two dogs who are old and terminally ill at the same time. This can easily happen if your dogs are close to the same age, and it’s a financial, energy, health, and emotional burden on the humans.
If you want to participate in a certain activity with a dog—a dog sport, regular walks or jogs, search and rescue, therapy dog visits, or something else that requires the dog to be physically sound—having dogs who are old at the same time forces you to either stop the activity until one of them goes to a heavenly reward, or add more dogs than you had in mind. That can take away from both the bonding and training to the new dog and the loving care you want to lavish on the precious old one as the end approaches.
Spacing the dog’s ages as much as you can has advantages both for you and for them. Five years is a nice age difference for a two- or three-dog home.
People often get a second dog to keep the first one company. You can provide that company yourself. Besides spending time with your dog, you can arrange play-dates with compatible other dogs who also don’t have housemate dogs. A good place to meet prospective playmates and their owners is a training class. Before getting a second dog, you need to observe your dog with other dogs, and you need to train together to the point of off-leash control. You need to know if this dog even WANTS a housemate. Some dogs emphatically do not.
Before adding a second dog, work through or figure out how to reliably manage any behavior problems your first dog has. This includes separation anxiety, inappropriate barking, aggression at windows or fences, killing cats, housetraining accidents, and other such problems. All of these behaviors easily spread from dog to dog when they live together. Two dogs doing any of these things can be more than twice as difficult to live with as one doing it.
If you want to pursue an activity with your first dog, get well down the road into that training and participation before adding a second dog. Otherwise, it’s highly likely your first dog’s training will suffer and the dog will never get to live up to his or her potential. The second one won’t do as well as possible either. What you learn with your first dog will profoundly benefit your work with the next one. All of you will be much better off if you wait until the right time to bring in another dog.
The whole article gives a lot more detail on getting a second dog:
http://www.veterinarypartner.com/Content.plx?P=A&A=2465&S=1&SourceID=47
Incidentally: I am not sure where you are based, but most good, health and temperament-focused breeders in the US and Canada will not home cavalier puppies at only 8 weeks of age as they are so immature and small. The norm is 10-14 weeks. At that age, a puppy is also far easier -- more confident, has started on housetraining, and has the great benefit of beginning to learn bite inhibition and polite behaviour from mother and siblings. So whatever the case, I would not consider bringing home a puppy til 10 weeks anyway.
If you are mainly getting a dog as a companion to a dog home alone, the older the new addition, the better. It is never safe to leave a puppy at home with an older dog -- you will have to keep them separated for many weeks and someone is going to need to get home every couple of hours to housetrain the puppy. Do consider that if no one is home all day, adding a second puppy is going to be a huge amount of initial work as you will also need to give equal and separate time to the existing dog (two dogs are like two kids -- they need double the time, not the same amount divided between the two, and puppies need about 5 times more time and attention than an older dog because they are only starting to learn). In a working household I would recommend a young dog of at least 6-8 months; an adult over one is even better. Given the age of your current dog, I'd be looking for a 3-4 year old as a good mix for the reasons noted above, rather than adding a young puppy.
It's really great that you are asking questions and seeking advice before you make a commitment as it is a big decision and timing can make all the difference between the second dog being a burden or a success.
My one dogs are 2,4,4, and 8.
I STRONGLY recommend insurance with this breed and make sure the insurer covers inherited problems (only 1 or 2 do). This is a breed that has two widespread and potentially costly health issues: mitral valve disease and syringomyelia -- and most dogs will have both or (with SM) aspects of both by the time they are mature. Also any problems with patellas or eyes can be very costly as well.
