Two things:
1) It seems like having both NPCs on Furrow set to harvesting more items does not increase the 'cap' of how many items a seed can yield. In other words, if a seed can yield 6-10 items, it won't make the seed grant 13 items. I tried this over the course of several weeks using Tree Saplings fertilized with Garden Worms(+50% Bounty) and both NPCs set to increase harvest. I still only got 8~10 items. It probably just increases the odds of getting the max without increasing the max.
2) In terms of ranking up Furrow, the number of items per harvest is important to consider. Went setting the Mithra to "Look for Seeds" it guarantees at least 1 of the items received to be a seed, but like it seems to increase the yield too. So for Revival Tree roots, without any fertilizer or NPC increasing the harvest, I'd get 2-3 items but with "Look for seeds" i'd get 3-5 items with 2-3 of them being seeds. So it seems like setting the mithra to harvest seeds or get more items, either way it will increase the overall number of items you get. So for instance without the mithra, a seed may give 1-3 items, with mithra it will be 3-5, but the only difference is if you set to "look for seeds' it forces some of the items that would normally have spawned(acorn, moko grass) into seeds. Cuz even when I don't set her to increase my harvest but set her to look for seeds, my average bounty seems to increase by the same # of items as I'd get if I set her to increase crop harvest.
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