With a simple setup like the one in the picture with a double chest connected to 2 hoppers, you can get stacks of items evenly distributed. In this setup half would end up in each single chest.
In my single player world (1.12) I have made a sorting system and I want half my cobblestones to go to a smelter and the other half to go to a storage chest. My sorting system has hopper lines going in from all my farms, mines and so on so all my items pass that way. Using this mechanism worked really fine during testing, I plonked stacks of stone into the input chest and I could add more chests and hoppers to divert ¼ or other fractions as I pleased.
However, when I hooked it all up to my supply line it only sent items to the first hopper. I realized the problem is that the double chest only receives one item at a time so it will always get sent to the same hopper.
Is there a simple way to remedy this? Can I make it retain items until there is a stack of 2 before sending them to the double chest?