Main idea: 
- Disallow a marble to get stuck in between tracks by using a spike and gravity
- Allow a marble to pass next to the top marble of the other lane by adding a sloped channel
(see image)

Designed for 16mm marbles but due to parametric design should be easy to modify for other sizes. To modify it to your needs change the marble diametre and tweak the "marble_pass_length" (size of the side channel at the junction) parametre to obtain the wished behaviour.

Not the holy grail yet, but could probably become it with some tweaks. Only 1 position results in a temporary jam, otherwise perfect (about when a marble lane is offset by ¾ marble diametre).
Not designed to prove Martin wrong but to try and help at my level.