Dual drive is a good set up. Instant gross shifts at lights etc. You will struggle to notice the drag. The only thing is I don't thing you get the full benefit from a birdie as they are unstable to steer at really high speeds so the upper ratios may not get used.
I run three 8 speed birdies and find enough range for offroad use. However, one of the benifiet of dual drive is shifting with either hand in to other gears at lights and during braaking.