Originally Posted by
fietsbob
You mean you want to shift your rear derailleur with your left hand ,
and put the right lever on the left side of the bar?
Nope, that's not it. What the OP was considering is often done on bikes that have the rd's chainstay cable stop under the stay and downtube mounted housing stops. You install both levers in their normal location but you run the shift cable housing to the opposite side of the frame (left brifter housing to the right side downtube cable stop and vice versa). Then you cross the bare cables under the downtube and run them to the normal derailleur. That keeps the cable housing off of the headtube on tight turrns and avoids rubbing off the paint.
Many modern frames have the housing stops located very high on the downtube or on the headtube itself so the cable rubbing isn't an issue.