This is one of nine little panels that used to sit along the bottom of a much bigger altarpiece, for a church called San Marco — they've ended up scattered across different museums since, from what I can find: one in Paris, a few in Munich. The story behind it: a woman named Palladia was healed by these two brothers and tried to thank one of them with a gift, which apparently started an argument between the saints over whether it was okay to accept it.