You do one at a time if it is variegated, e V ennis it is V one color variegated, for the same reason. Also because you don'to get an odd look frommit. But, WDW has flosses that are not variegated. You can stitch those just as you would stitch any solid colored floss...
I am a big fan of their floss because it truly is colorfast and requires no pre-washing, rinsing, etc. Saves a day of prep time!