My split boards have a controller in each half. That is they can be used separately as keypads.
Also they can be programmed separately: one half can contain the default layout while the other contained some experimental/test configuration. So the keyboard behavior would depend on which side it is connected to computer with.
I always new that - used to play with the layouts as described above.
What I did not realize - one of the halves could be configured to work with an online configurator (REMAP/VIA) while the other be configured with the regular compiled QMK configuration.
I accidently found that today while adding VIA/REMAP support to ThumbsUp! v7, so wanted to share this discovery.