With downtube shifters, yes, either shifter with the right hand. Often I'll put my hand down there, shift the rear, and feather the front right before bringing my hand back up. Or vice-versa. Sometimes I practice doing this with my left hand, but it feels clumsy.

You may be doing this more than you realize. When I started riding bar end shifters about a year ago (I'd used them before, but not for a long time) I often found myself reaching down to the right bar end to feather the front derailleur. It didn't work, and I'd be confused for a moment before remembering the front shifter is on the other end of the bar. Bicycling tends to shut down the rational part of my brain.