Double triggers are not in-line, in spite of having been described as such, in a Lefever ad that was on Gunsamerica for a long time. They are in side-by-side slots and work well that way.
I shoot SxS guns mostly and often shoot the rear trigger(left barrel) first on doubles. After shooting I have no memory of moving my finger. The flintlocks and early percussion doubles had very short front triggers, presumably so your finger could slip underneath to get to the rear without moving it outside the trigger guard.
