Dogs who are friends will chew on each others necks as a form of play: if either dog is wearing a collar, it can become a casualty. You can work with your pup to curb the chewing tendency, but some dogs are naturally more aggressive chewers than others.
Provide plenty of toys for your dogs during playtime. Like small children, dogs can make toys out of anything, and their choices may not be ideal from your point of view. Many dogs like to play tug-of-war, so a long rope bone they can use for this game will often keep two dogs occupied until they wear themselves out.
Apply a taste deterrent to the collar. Commercial taste deterrents are widely available at pet stores and very effective. If you apply the taste deterrent regularly to the collar, your dog will quickly learn to avoid putting his mouth there.
Separate the dogs if they get too rowdy. Some dogs, no matter how well they get along, cannot play together for extended periods without getting too rough. While the game may seem fun to the dogs, if you believe they are being too rough, separate them until everyone calms down.