Here's a sketch of what philly is talking about.

There are some practical advantages to v-tails, like only building 2 surfaces, keeping the surfaces out of the grass on landing, etc. However, the idea that there are efficiency gains is largely myth. The only aerodynamic advantage that I've heard of is that the tail cannot be completely blanketed by wing wake on a quickie racer.

People that claim to see a difference often have built the v-tail smaller than the equivalent conventional setup - then claim less drag.

It's a cool style choice though, so if you want one - make one.