there is a very small window to use a flurry proc before the ebonbolt procs takes over, even if you already have a flurry proc while casting ebonbolt you can use it straight away and you still get the ebonbolt flurry proc.
If you have a flurry proc already but are casting ebonbolt, you need to use flurry straight away and you will still get your proc from ebonbolt.