MoM's that have a hitter or a thrower

The past few weeks have been rough in our house. One of my guys has started the ever dreaded hitting and throwing stage. I've been doing everything I know to do, enforcing that hitting hurts, sitting him down to calm down if need be etc. The thing is, I understand its a phase and have complete patience for that, except for the fact that he's doing it now to his brother. And although they're twins, "b" is much smaller and daintier and passive than "a", and he's getting beat on constantly and it's just breaking my heart. I was changing "b" the other day and "a" came up and whacked him in the face with a toy just barely missing his eye before I could react and stop it. Then tonight I was cooking dinner while they were playing and hear "b" scream so I look over and "a" is whacking him on the head over and over with a toy. Hes also been throwing toys (very hard) at us, brother, the dog etc. If it was just me or dh I wouldn't be as worried but it just kills me to see him hurting his brother. Have any veteran MoM's dealt with this and have any words of wisdom?!