It’s a two-step process: you have one flow that will trigger when it’s time to start the wake-up routine. That flow will create a new transition that will, say, take 20 minutes to increase a value from 0 (= dark) to 1 (= fully lit).
Next, you have another flow that will take that value from the transition as trigger, and set the brightness of a set of lamps in its action.
For more information, look at this post on the forum. It contains something similar to what you are trying to achieve (although it only uses one lamp).
The README contains some more information on how the “short form” of creating a transition works. It’s not necessary to use it, there’s also another action card where you can fill in each value separately, but the short form is quicker once you understand how it works.