The initial design for the doorways had functionality (and even a model) for doorways that intersect and even vertical ones.
This was never implemented due to complexities / overhead of encoding this in the block itself.

Since the initial version was for Minecraft 1.12 and the particular limitation has since been removed, it may be time to revisit this.

Closing as done. Doorways can now cross.
Vertical doorways are covered by #90