Sideloading belts in (current experimental builds of) 0.16 tends to cause them to lose compression. If it's desirable to feed two different materials to a production unit, this can be troublesome. This is a workaround -- feed each input splitter with at least half a belt of that side's desired item, and the output should be a compressed belt.
It's not particularly compact or elegant, so hopefully this will become unnecessary again in a future patch.