Well big ag was supposedly able to solve it right? Vertical integration helps coordinate I suppose, but I don't see how it should be impossible without.

So there's generally an optimum time of year for each step, and being a couple weeks early or late can definitely lower yield a bit. Big agriculture can stagger planting so that a smaller number of machines can be used per acre, if that's more profitable overall, even accounting for the slight reduced productivity. A group of families can't do the same thing nearly as easily if it means that one family makes a large profit (they hit the optimal time), another family makes a small profit (a week or two late), and the unlucky family possibly makes no profit at all (even later/earlier). In order for this to work, they'd also have to share profits, and at that point they've basically just merged.

I think there could be other solutions than outright sharing profits. I'm thinking something like higher tractor-rent during more desirable weeks. I don't know a lot about farming but I think there is also a risk management thing that needs to be dealt with: Planting earlier or harvesting later than optimal means higher risk of crop failure. This could be handled with insurance.