Exporting a BPM synced Rekordbox playlist to USB

Umm... tracks in your playlist are sorted by a default order, typically the order in which they were added to the playlist. You can re-number the playlist if you sort the list in another method (eg. by BPM), simply click the Playlist menu item, then click Renumber Track Order.

The numbered order will then be updated - and you can then export that to your USB drive and the playlist will remain in that order. But don't forget, you can also sort on the CDJ-2000.

If you're strictly talking about whether or not two tracks will be in sync when they're loaded to CDJs, that depends entirely on whether or not you have sync enabled on the players at that time.