There are two definitions for this term:
1. Demibigenderflux is an identity describing a gender that is partially, but not fully, bigenderflux.
Demibigenderflux combines the concepts of demigender (partially identifying with a gender), bigender (identifying with two genders), and genderflux (identifying with a gender which fluctuates in intensity). It is also similar to demigenderflux, bidemigender, and bigenderflux.
Someone who identifies as demibigenderflux experiences two genders, both of which fluctuate in intensity, between agender and demigender. In other words, a demibigenderflux individual experiences two gender identities, each fluctuating in intensity, up to partially identifying with that gender identity.
For example, identifying as both demigirlflux and demiboyflux. These two gender identities could be experienced at the same time, or fluidly. Demibigenderflux people may feel like agender or libragender often, not necessarily identifying with demigenders all the time. This label describes the maximum intensities that a person's genders are experienced at, but those genders will not always be at that maximum intensity.
2. Demibigenderflux describes part of one's gender identity being bigenderflux.
Resources
https://archive.today/https://www.deviantart.com/pride-flags/art/Demibigenderflux-2-870952138