Excepting unusual conditions, most nearby farmers needing the same equipment will need it at similar times. You can get around this by transporting machinery long distances as migrant workers do to hit multiple harvests in various parts of the country, but that gets expensive.

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.

In Germany you can rent equipment and operators for this equipment. A lot of farmers do it for crops they don't usually harvest but are required to to keep the land fertile or to collect subsidies.

The problem with this model is that, as the sibling comment already pointed out, harvest is done typically within one or two weeks or even in two days, depending on the weather. Harvesting too late can make the crop go bad. So renting the equipment and getting it to the field can be a hassle. It's still being done a lot but it's not risk free.

This situation has interesting second-order effects as well. Most "custom harvesters" require farmers to schedule their combine/truck well ahead of time, giving them little leeway in which specific dates they can harvest their crop. So instead of for instance letting the corn dry out in the sun, they'll spray an assload of glyphosate on it to cause it to dessicate to ensure it'll be at the right moisture level right when their custom harvester shows up.

That sounds much the same as the "contracting business where the equipment is owned by someone who does contract jobs for many smaller farmers" case.

Contract harvesting, sowing, spraying etc has been a thing for decades.

It's more economically efficient for used equipment to stay in use.