That is not supported at the moment, in part because the thing produced is then collected into a basket by the farmer, so that would add some extra complexity to the basket/gather mechanism. Probably conceptually what you want is a crafter of some sort who takes what is produced and turns it into 2 things, so plant -> stalk, stalk -> grain, straw. Having recipes produce multiple results IS supported.
Also, you can have crops produce different things at different stages.