Placing ghost underground of different color over another builds both undergrounds.
See video for demonstration
https://youtu.be/Ts6SlrcAMfI
[1.1.24] inconsistent underground belt placement over ghosts
Re: [1.1.24] inconsistent underground belt placement over ghosts
You can see how I discovered it in my twitch stream, https://www.twitch.tv/videos/916999784?t=03h54m20s