Hi
I was interested in creating some complex behaviours but something was not working. So I watched again this video https://www.youtube.com/watch?v=GU4aXyyBdIU to recap the basics and I’m doing everything right, or maybe not.
But if I reproduce these steps (clouds+boats), only the second timeline is played.
Any idea what am I doing wrong?
Thank you.
Even worse: when I don’t use the Custom Behaviour technique, but just assign two actions to the button so it plays both Timelines, only the second Timeline is played.
what’s your aim¿
the ‘RED’-Timeline hides the element animated in the maintimeline … so its not shown.
The aim is to show both timelines at the same time, as explained at the beginning of the video.
In the video, each timeline does not display the other element.
I need to have one timeline where we see only the gray circle for one animation.
I need to have another timeline where we see only the red circle for another animation.
The purpose of the video tutorial is to take these two separate timelines and make them play together so we see both items together, as with the two objects in the video tutorial.
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.
I perfectly understand that but then how do you explain on the mentioned video that when he plays the first timeline we don’t see the object of the other timeline and vice-versa?
I believe the timelines in the video are using the opacity property and fade in and then out as part of the timeline that is triggered.
Your document sets the display to be hidden for the grey Ellipse when running the Red timeline.
Timelines are like streams of animations, so if you animate the Grey ellipse to be hidden, that will get executed and remain in that state. The small detail to note is that simultaneously running timelines will not be allowed to animate the same property (as this would likely create a “ping-pong” effect each frame if you were trying to control something like position).
So here’s what’s happening: