Hi There,
I have a need to sync two parallel strips, but where one is shorter then the other and ideally where they act as one - not two identicals. Because I’m bad at explaining things, i’ll say it a couple times in different ways! 
These are two strips next to eachother… The X is to visualise where I want the sync from.
… … …
… … … … … … … … … … …
If a single LED (lets call it the bouncing ball) enters from the right hand side larger strip (Y) below and travels left, I don’t want it to illuminate on the shorter strip until it gets to X below…
…<…X
…<…X…<…<…Y
Possible?
As I’ve typed this, I wonder if I set the shorter strip to the same LED count as the larger one, thus lying to it and it may then work… but it would have to input from the far left which is a pain - maybe - oh reverse it! - thinking out loud … would that work? (haven’t got the parts to test yet).
Further question - Is this possible from a single controller?
Thanks!