I would take the remaining dog with me to a shelter and see how he reacts to other dogs again. He may help you "choose" a new friend, or he may stick like glue to you, giving you an indication that he's not ready yet.

I would ask neighbors if they saw anyone near your yard just before the incident - that does sound peculiar, and though nothing will bring your babies back, it would be good to know whether your current dog would be safe or not.Is your yard gated, where someone could have opened the gate, then shut it again? If so, can you get a lock for it?

Our hearts ache for you, sooooo sad, you have the right to grieve and we understand how big a part of your life the dogs were.