Imagine that you have raised the vee tails until they are vertical. Now you have the same setup as a twin-tailed aircraft like the P-38 or B-24. Note that both surfaces move in the SAME direction to yaw the aircraft. Then, in your imagination, lower them to the vee angle of your design. The trick is to remember that this is a yaw control and not a roll control like the ailerons. (From somone who got it all wrong!)