Kyatric can you help with that?
Well, after many searches and many attempts, i've figured out how to do. Actually it's very simple.
With your "to scroll" background (in this case a starfield) taller than your static background, you just set Layout properties> Unbounded Scrolling>yes
And now, the reverse sentence should work.
System on every tick: System set scroll Y to scrolly - 8 (or any speed variable) *dt
**I didn't implemented the loop yet, but should be not too difficult.
Thanks for the attention!