To solve the split brain scenario, you effectively want to require a quorum (i.e. a minimum membership size) before proceeding with a singleton master election.  This is implemented in WildFly.  You can probably implement something similar in AS 7.1.1 via a custom election policy.

e.g.

public class QuorumSingletonElectionPolicy implements SingletonElectionPolicy {
     private final SingletonElectionPolicy policy;
     private final int quorum;

     public QuorumSingletonElectionPolicy(SingletonElectionPolicy policy, int quorum) {
          this.policy = policy;
          this.quorum = quorum;
     }

     @Override
     public ClusterNode elect(List<ClusterNode> candidates) {
          return (candidates.size() >= this.quorum) ? this.policy.elect(candidates) : null;
     }
}

It's a bit of a hack, but it ought to work.