By default, Shift+1 for engine one, Shift+2 for engine 2, Shift+e to control both engines normally. Both throttle and RPMs can be adjusted individually.
A hammerhead is a vertical manouever with numerous variations. The basic principle is that you lead your opponent into a steep climb until he can no longer follow and begins to stall out, at which point you reverse and zoom down on him while he has little control of his aircraft. Proper execution takes good timing, which comes with practice. Your opponent must also not have more E than you, or you'll be the one to stall out first and get blown away.