Do players get rewards for farm co-op?

The work happens at somebody else's home, which means farm co-op starts with an invitation. Your main world is a private instance, so friends have to be let in before any of this is possible. Nobody wanders into your beds from a public space and nobody wanders into theirs.

Two jobs pay out, not one. You can till soil and you can harvest, so you do not have to arrive at exactly the right point in a crop's growth to leave with something. Show up early and turn over the beds. Show up late and bring the crop in. Either way your cut comes out of that farm's harvest, so there has to be a harvest for you to get anything at all, and a friend who never plants is a friend whose farm is worth nothing to visit.

The practical shape of this is a standing arrangement rather than a one-off. If you and a friend both keep farms and both visit, the labour goes both ways and so do the crops.

Not settled: How large a share a visitor takes, and whether the host's own yield shrinks to pay for it, is not pinned down.