Yvonne,
Since you decided to discipline B, then you have to bite the bullet and continue with it. To look at things positively, you are having hard time now but once he knows and understands his limits, it will then be smooth sailing for you.
While for parent like me, both me and hubby don't discipline HR. So there is no limit and no boundary for him. We use distraction method, and if it fails, we let HR take the lead. We enjoy our time with him a lot now, but then, it may hit us hard when the time comes when he becomes out of control and it's very difficult for us to start disciplining him then.
Just hang in there, with whatever decision you have made!
On a lighter note, that day when I talked to my hubby, I raised my voice, but we were joking. But HR looked at me a bit shockingly, and when I stopped talking, he said "妈妈坏蛋". He thought I scolded his daddy

Everything we do now, they are observing and learning!