I'd expect there to be three episode scores on a typical disc. If the scores are arranged in production order, then "Adonais" would be fourth, "Amok Time" fifth, "Doomsday" sixth, and "Mirror" seventh.
I was thinking last night after I posted, that my estimate of what scores could be on disc 2 was based on the idea of the scores being in series order. Your post mentions production order. Neither one of those is necessarily the obvious choice.
Returning to Jeff Bond's indispensable book, the scoring schedule is on page 33. Why wouldn't that be the obvious order for the CDs?
Scoring schedule for the first part of season two:
Friday's Child 7/7
Amok Time 7/19
Doomsday Machine 8/30
Mirror Mirror 9/8
Bond lists 34 episodes with original music. We've learned during the runup to this release that that number may not be 100% accurate – there is at least some new music in Way To Eden, for example – but it's pretty close. 34 episodes over 15 discs gives a little over two scores per disc. But not all of them were full scores; some were full and some were just part scores. I think Kaplan's score for Enemy Within was a full score, so I'd guess it would take up half a CD. But Steiner's scores for Corbomite, Balance of Terror, and Little Girls were all part scores – in fact they were all recorded the same day. Those three probably combine to take up another half CD.
This notion that a day of recording gives about half a CD is supported elsewhere in Bond's book. Bond quotes an article Steiner wrote for Wonderful Inventions
. Steiner writes that the ratio of usable music to performance-recording time was about 4-1/2 to 5 mins per hour. He says that the Charlie X score was about 21 mins of music, and that would have taken "not less than four and a half hours" to lay down. It all fits:
average recording session ~= 4-1/2 to 5 hours --> about 18 to 23 mins of music score ~= half a CD
Bond lists 30 distinct recording dates. That matches 15 discs, perfectly. So that's going to be my guess: the set arranged in scoring schedule order, with two sessions per disc. That will usually give two episodes per disc, but sometimes more where there are part scores.
I suspect "scoring schedule order" and "two sessions per disc" might break down in one or two places. GNP Crescendo tried to put the two pilots on one disc, and we didn't get all the music, possibly due to space constraints (those are big scores). So it wouldn't surprise me if those were split, each paired with slightly shorter scores. And somehow Duning recorded the Truth No Beauty and Empath scores on the same day – those are both full scores, right? – which must be some kind of record for efficiency. Wouldn't surprise me to see those two scores split, each paired with a shorter score. Maybe Spock's Brain paired with Truth No Beauty on one disc, and Empath paired with Plato's Stepchildren on another. That preserves scoring schedule order.
Going back to the scoring schedule – if Neil was listening to season 2 disc 2, then it may have been Friday's Child and Adonais. A good disc.
Oh, and note that if recording schedule is used for the order of the discs, then season 2 disc 3 preserves the GNP Crescendo pairing of Amok Time and Doomsday Machine. Another good disc.