It's easier to switch the side of a brake lever than a shifter. Also, as others have said, the front brake is the more important, so ditch the rear one first. Therefore, with a front brake operated by the left hand and a rear shifter on the rigth side, there is no longer a problem.

Another solution would be to not use handlebars that are too narrow - with some regular-width handlebars, there should be plenty of space for shifter and brake on the same side, and you won't look like a dumb hipster who is obsessed with riding with his hands almost touching each other.