Maybe this is working as designed, but I think it could be better. I use side-loading on underground half-belts (one side of a pair of undergrounds) to create lane balancers. I then place opposite direction these balancers in close proximity. I am required to offset these balancers by one tile so they do not connect. I believe one should be able to use opposite direction undergrounds as I am intending without having to use an offset work around.
My picture shows the balancer as I would like to use it and the balancer as I currently use it with a work around.
SUGGESTION: Perhaps this sort of thing could be solved by creating a new entity: the side-loader. So frequently we have to wiggle belts around to get our products to the desired side, and we are using underground belts for something other than intended to get it done. Imagine an entity that functions as a single underground, allowing (possibly configurable) side-loading without having to worry about connecting to other entities.
[1.0.0] Opposite direction underground belts not allowed
Re: Opposite direction underground belts not allowed [1.0.0-0]
That is not a bug.