You've got a number of issues there and an anti bark collar is a very poor option and may only make all of them very much worse! It is punishing behaviour without finding the cause. A vet visit would be the number one item on the agenda to find if Charlie has some medical reason causing him to snap and growl -- this is not normal behaviour for a cavalier. Also if you only recently got Max -- it can make a resident dog very upset to suddenly have to share his home and family with a stranger and dogs do not always like having a companion. Charlie may be a dog who is better on his own. On the other hand he may need at minimum a few weeks and even many months to adjust to a new dog. You need to work with the situation to help a resident dog accept and adapt to a new dog -- I do have some good links in the ibrary section on adding a second dog to a household.
Have you done any formal, rewards based obedience with Charlie? This would likely help too, and if you've never done any training with him, would be an urgent necessity. As is, I'd at least recommend getting in an APDT certified behaviourist (find one at
www.apdt.com) to work with you on the issues you are describing as this is quite serious and you are right and responsible to be concerned about the snapping, especially why it has started out of the blue. In the UK you could have to have Charlie forceably put down if he bites someone who files a formal complaint, so it really is an urgent issue. This is very unusual behaviour for a dog this young. A bark collar would be the worst possible thing you could put on a snapping dog especially if it is pain causing these reactions -- it would just be unbearably cruel.
On the more difficult side I would consider rehoming Max if he seems to be the spur to all the unwanted behaviour. Adding in dogs can totally change the dynamic in a house and can definitely start unwanted behaviours.
I recommend as well getting one of Dr Ian Dunbar's books on owning dogs -- no home should be without a good manual as it will answer many questions on behaviour and training issues.
Just one thing to consider -- most trainers do not advise getting a second dog til the first is at least 12-18 months old, and has been well trained. Otherwise you can end up having a very hard time training two, and the first dog gets neglected during an important bonding and training period because the second takes up a lot of time and interest, especially if it is a puppy. That alone may be causing the problems. Hence the advice to get in a good APDT trainer for professional guidance, and perhaps to consider in consultation with a trainer, whether it is wise to hang on to Max under the circumstances.