I have some suggestions for a couple of things you mentioned.
All you need is a small piece of PVC pipe in order to let them play together without destroying their very expensive electric fence collars. Cut the PVC as wide as your collar, cut a slot running the length of the pipe (to allow the electrodes to still make contact with the skin), then simply slip the pipe over the collar. I can take pics of mine and post them when I get home if this description isn't any help. This has prevented our meathead pitty from destroying any more collars. Also, if you have plastic buckles on your collars, swap them out for ones with metal.
In terms of a more destruction-proof toy, freeze a bucket of water. Dump it out of the bucket and you have hours of fun for your dogs. The added bonus is that it's a calorie-free treat/toy. Our dogs LOVE ice.
db