Don't let your dog out unsupervised. Walk him on a leash or put him on a tie out and watch him if you know he can escape the fence.
getting a second dog, when you are having trouble keeping the first dog properly and safely confined would be incredibly irresponsible. additionally IF the dog's issue is separation anxiety, getting another dog is NOT a good fix for most dogs.hmmm...sounds like your dog might need a doggy friend to play with (as well as being contained completely) GET ANOTHER AUSSIE OR A COMPANION DOG.
We did that for our dog Chloe...and rescued another catahoula....Chloe and Addison are inseparable and it completely decreased any separation anxiety, helps with their exercising...and is double the joy!!!!! Some may not agree with me to get another dog...but there are a lot of dogs needing homes!!!