TL;DR
Implement a lane splitter building to give us better control over the individual lanes on transport beltsWhat ?
Add a lane splitter building 1x2 tiles in size. It's a specialized section of transport belt (like underground belt and splitter). It attempts to redistribute items between the lanes of the transport belt. The items from each input lane will be evenly distributed to both output lanes. If one output lane is blocked, all the inputs will be routed to the other output lane. A lane filter also has an output priority setting with a filter that works like that of a belt splitter.The lane splitter is primarily a lane balancer but it also has many other uses. It's possible to replicate this functionality with existing belt splitters but the result is ugly and relatively large. A small lane splitter could be used in compact assembly lanes in ways that the belt splitter based solutions cannot.