I’ve got the same setup. XSV on bars, X2 on helmet.
I originally had a sync issue and they would get out of sync several times per ride (VERY annoying!). To get them back I would have to press the button on the light (not the remote) to get them back into sync. This was when usoing the sync mode where the buttons cycle through the brightness modes (e.g. dim, medium, bright, then back to dim, medium, bright etc etc. Same as how the button on the light unit works).
I fixed this by changing the sync mode on the remote. I now use the mode where the up button makes them both brighter and the down button makes them dimmer (rather than the endless cycle that get out of sync). That way, if they ever get out of sync, I just press the up button several times and they both go back to full brightness and are in sync again.
I hope that makes sense…
Having said this, since changing the sync mode, they have never gone out of sync (not that I’ve noticed).