The best way to handle it is not to. Don’t agree or disagree, don’t discuss, don’t offer suggestions or commiserate. Your responsibility is for you to get through the workday. Your job is your job, which I assume is not to be A’s sounding board or to hound B for slacking. If A keeps bitching about B, ask A not to. If A asks you why, be honest and say that the constant complaining is stressing you out and you’re not the proper outlet for this. If B needs to pick up the pace, that’s for her manager to deal with.