TL;DRSplit belt: used to split the 2 lanes of a belt and send them off to opposing directions
What ?A new belt (in different belt tiers) that is 2 belt ends going off in opposite directions:
Why ?In the game it's trivial to combine belt lanes into a single lane by side-loading, but hard to split the 2 lanes again. Personally I dislike the underground belt "solution" and consider it more of a hack.
A simple, elegant (IMHO) solution for this would be to introduce a split belt that goes in 2 opposite directions. You could think of it as 2 belt ends shifted by half a tile. Doesn't require complicated mechanisms like smart splitters, doesn't leave half of an underground belt pair unused, doesn't block items on 1 lane of a belt (as, again, with underground belts).