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.
Wheat (in real life) is a good example of a crop that can be harvested twice. Once for the wheat and once for the straw (for brick making as well as many things). Perhaps some tech change is needed to be more accurate?
I’m sure there are other crops that do the same thing. Oh yeah, pumpkins. We harvest the flowers and eat them and then the pumpkins from the unharvested flowers. Love me some pumpkin flowers.