Perhaps if one dog had a proven history of prior attacks it could establish liability. However, more probably both owners are equally responsible for allowing their dogs to be off leash. You could consult with an attorney who handles animal law matters.
This is not intended to be legal advise or as legal representation. I am a California personal injury attorney . Be aware that every state has its own statute of limitations; and statutes & case laws that govern the handling of these matters.
That depends on more complete information. I won't guess. I certainly would like to know if either dog has a record with Animal Control. I'd like to know the training, supervision, and bite history for each dog. And, I'd like to know each owner's behavior during the incident. City and County laws might exist and bear upon liability.