You can't use absolute numbers. X is unlikely to be exactly -960. That's wrong any way. Use only two backgrounds, and change it to:
+ bg1: X < -5000
-> bg1: Set X to Self.X + 10000
You want to make sure the first one is off screen, then adjust by double the size, again not absolute, but relative.
Edit:
Or better yet, use bg1.Width, so there are no fixed numbers.
+ bg1: X < -bg1.Width
-> bg1: Set X to Self.X + Self.Width×2