Threatening to walk out of a meeting if the dynamic duo decides to use you as a punching bag again is simply letting them continue to get away with this nonsense. The fact is that this behavior has to stop, and that is what you need to tell both of the perpetrators as well as your department head. If your department head cannot bring it to a halt, perhaps there is someone above this person who can.
The fact that one of the perpetrators is a buddy of the manager certainly does not make this situation any easier for you. However, the real problem is not the two attackers, but rather the department head. When there are attacks in a meeting, an effective leader stops them and then follows up with a combination of discussion, mediation, and counseling to prevent these outrageous acts in the future.
If the problem continues, you should look to senior levels in the company for help. Unless the topsiders are totally out to lunch, there has to be someone up there who realizes that only weak leaders tolerate bullies.
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