were the dogs ever properly introduced? Did you take both of them for an hour+ walk before putting them in your house? Did you separate the mastiff under house quarentine until you are comfortable that the dogs will be ok together? And of course you did introduce the dogs on a neutral territory right....like down the street or at a different location than your house?
You need to introduce the dogs appropriately, not throw them together in the back yard. You asked for disaster. And because you admit it is your fault you want to "get rid of him" then? Just throwing them into your home right from a shelter is like throwing him back into a shelter. I am sorry if you take this in the wrong way, although this dog depended on you, and you couldn't do it...and if that is the case you should have thought of that to begin with rather than tossing this poor mastiff around from home to home. I am flabergasted that you are the one at fault, and because of that, his life and well being is up in arms now. How can you do that to him? You need to try again, it isn't fair to the dog. Being a mastiff owner in MA, i am apaulled by your post. And as i said, sorry, but you deserved a dose of reality. (the worse part here is that the shelter gave you this dog with out meeting your personal dog first. On top of that, you didn't know how to intoduce them properly. TRAIN YOUR DOG...ignoranc e is what kills dogs...)