How to have a Fibaro dimmer match another device’s dim level?

[GSzabados (Gabor Szabados)]

You can group lights as it was described, but then you need to change on the group the brightness. If you change them individually, then they will have different values. But there are some old SmartApps intended for this

[OBSOLETE] Synchronized Dimming: a many-to-many Dim With Me

This is a simple app that synchronizes multiple dimmers, such that any one dimmer changing will change all of them. It differs from Dim With Me in that no virtual dimmer device is necessary to tie multiple dimmers together bidirectionally.

Or you can use WebCoRE or Sharptools.io 's rule engine to define, if a changes, then change b too. Etc.
There are multiple ways to do similar things.