The first trainset (2x00) was delivered with an extra loco - 2201. Eventually the locos were swapped for whatever testing was necessary, and that trainset remains configured like that today (2201 with 2300).
All of the subsequent trainsets up to 2x08 thus were delivered with a loco numbered one higher than the trainset (2202 with 2301, 2203 with 2302, etc.)
Then trainset 2x09 was delivered without a loco. This was to allow them to finally put a train onto that one spare loco. (The assumption is that the numbers will be shuffled to allow the trainsets to all match numerically, but I've also not gotten confirmation of that.)
And since then, matched trainsets have been delivered. The 16th trainset was delivered last week, and the 17th has been booked to be picked up from Siemens in Sacramento at the end of May.
Dan