I would also say the worst thing that can happen when you mutibox PvP is getting hit with a fear bomb.

Also, mutiboxing doesn't remove the class weaknesses. Warriors are weak vs mages, warlocks weak vs rogues.. etc.

I started by dueling with only using one hand over and over, and adjust keyboard and macros until I started winning regularly. Once you are comfortable add in your second account and start pvping. Practice and more practice.