I think You have a good understanding of Custom Behaviors - but not as good for timelines…
While timelines are discrete entities they are also an integrated part of the whole. What You do in one timeline can affect other elements in another timeline.
This is what @h_classen was referring to when he wrote:
Let’s review the sequence of your demo project:
Two timelines: “Main Timeline” & “Red” timeline.
“On Scene Load” timeline - “Main Timeline”.
In the “Main Timeline” You have the grey circle visible (“Display” property set to “Visible”) - the red circle is not (“Display” property set to “Hidden”). This is the opening view.
Then we click the “Button” which starts both timelines running in sequence.
The “Main Timeline” starts running first - BUT then…
a micro-second later the “Red” timeline starts with the “Display” property for the grey circle set to “Hidden” and the red circle set to “Visible”.
End result: The grey circle immediately disappears red circle becomes visible & animates.
If You want both the grey & red circles to be visible and run simultaneously then don’t hide either of them in any of the timelines.