Firstly, this only applies to players who use two alts to afk their farms. As you know the maximum tax you can pay on your cropper farms is 32% and this is based on the number of croppers you have. Splitting those croppers over two accounts will obviously keep that tax rate down and splitting them 50/50 is the best way to go….. to a point. Without going into the maths of it all, I can tell you that that point is 54 croppers. So if you have 54 chunks of farms or fewer, split the croppers evenly between the two accounts. As soon as you get to 55 or more, you want to have it so that 1 account owns 16 croppers and the other owns the rest. This assumes that all chunks are earning the same which should roughly be the case if they are all the same size and same crop. Different farm types add too many variables for me to bother with. That being said if you are in the process of converting from melkins to wheat, then you want to first convert the chunks belonging to the alt with the fewest croppers and pay the lesser tax on the farms producing the greatest income.
A graph to illustrate your profit (revenue-tax) when varying the number of crophoppers on one account (given you're using two accounts) Desmos | Graphing Calculator
Nice. It shows the same tipping point of 54 croppers (27 per alt). After which, you want to have one account with 16 croppers and the other with the rest.