Typically, their voice changes several weeks before the drake feathers appear. I think the earliest I have heard a quack was in the 4 week range, but mostly it took around 6 weeks. Sometimes it can take longer to be sure though, since the drakes are sometimes quiet and it can be hard to tell who is quacking.
Luckily, with only two ducklings, it narrows the candidates down and should be quite a bit easier to tell.
Luckily, with only two ducklings, it narrows the candidates down and should be quite a bit easier to tell.