I did notice some ordering issues, though it definitely ended up with List A on top and List B on the bottom.
I suspect it is because ~seq adds sequence numbers, as I mentioned, as the files are
added to the resulting Sync List. So, it is going through
your entire Library, less any files excluded by previous filters, to see if they qualify and giving each file a Yes or No. It does NOT open up Playlist A, and then add those, and then open Playlist B, and add those. It goes through every file in your Library and checks them all. It does this in a particular order, but I don't know what that order is. Perhaps by File Key in numerical order?
It does work to use the OR between the lists to keep
those in order, but when filtering one list against another, I don't know if you can keep the order of the lists themselves precisely in order.
In my tests, though, the stuff at the bottom wasn't wildly out of order (tracks on albums were still, generally, grouped together, I think). The stuff at the top (from the Files to Include) list, I noted,
were out of order. All of my Music was on-top of my Audiobooks in my test list, whereas in my source "Files to Include" list I had them sorted by [Media Sub Type] and Audiobooks were up top. Though I think the different books and albums were still
generally together (perhaps tracks out of order, but who cares if they're all there).
I didn't check it extremely thoroughly, as I thought it was "within the margin of error" for your requirements. And, unfortunately, my test lists are all broken because I didn't follow my own advice and:
Be aware that you CANNOT CREATE OR MODIFY these lists (since they contain references to other Playlists) while MC is serving a Library and has any other copy of MC connected (even if the client is idle). Otherwise, it breaks those lists. Once you've made them, it is fine to use them with a client connected (or even from the client, though I don't know why you'd want to) but if you need to tweak them, you have to kill any connected clients. Because of the bug.
Yeah, I had the HTPC copy running upstairs when I did my testing for you, so those test lists I made when testing for you are all obliterated. I guess I could copypasta them back and verify. Maybe later.
But, I don't think that is solvable. If you have two lists, and you want to filter one against the other and have them be in precisely the same order, I think you're out of luck. But getting List A to always be on top of List B, you can do. Since List B is clearly of "lesser" importance (and randomly generated) I figured any shuffling or disorder is of limited consequence. A few of the albums that otherwise wouldn't have been might end up "partial", but when I looked at it, it wasn't a TON of them.
Perhaps this, though, depends on your original Library's data set? If it is by File Key, then maybe mine generally group simply because my Library is OLD. So, albums ripped together or downloaded together have sequential file keys, roughly. Where you imported your whole Library at once only a year or two ago, so it got File Keys based on whatever crazy optimized filesystem reading logic Auto-Import uses?
I don't know. Were you seeing a complete and utter
shuffle at the bottom of the Sync List? I certainly wasn't for either "Portion A" or "Portion B".