Over 20 years ago I invented a sight (US pat 4,597,211, now expired) that solves your problem by having a separate rear sight aligned for each barrel, alternated on firing by the inertia of a weight.

Maybe you could whip up a simplified version, perhaps using a folding Lyman sight with a second leaf Araldited at 90 degrees on the back. If you read the patent, you might get an idea.