i use a recipe that calls for double buttering madeleine molds. it specifies brushing with melted butter, freezing, and then rebrushing. it is one of those diminishing returns things. i don't always do it but if i'm cooking for dear friends--or the kids home on holiday--and i want to feel as if i've done everything i can to make a good madeleine, then i do. the increment in personal comfort is probably larger than the incremental improvement in taste or texture.
try using melted, slightly cooled butter for the second round. you can drizzle it in, let solidify, then sugar again.
FWIW, i know the *original* recipe says to do that, but all the reprints i've seen published over the years only say to do it once, and i don't think they'll really suffer if you just do one thorough pass.