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I've been using Serato for years and oddly, "Sync Start" is not a function I'm familiar with, and Googling that term didn't yield any results. Regardless, the easiest way to ensure your tracks are in sync with rekordbox is to have one of the decks set to sync as the master, and press the SYNC button on the other deck, then they'll both be in sync, even when you load another track.
Yes, there is Master Tempo on the DDJ-400; just press and hold the BEAT SYNC button:
