Thanks to Anonymous for making me track down a R' Yosef Engel in Tziyunim laTorah (#38) that deals at length with the issue of overlapping kedushos hayom. One of the gemara's he cites relates to our issue of rosh chodesh on Shabbos which we discussed last post (part I here). In discussing which takes precedence, tadir or mekudash (Zevachim 90), the gemara tries to bring proof from the fact that the korban musaf of rosh chodesh (mekudash because it is called a moed) is offered before the korban musaf of shabbos (tadir, because it is brought every week). The gemara rejects this proof -- does rosh chodesh effect only its own musafim and not those of shabbos as well? In other words, on shabbos rosh chodesh we are not dealing with the usual korban musaf of shabbos that just happens to fall out on the same day as the musaf of rosh chodesh (a harkavah shichnit), but rather the kedushas hayom of rosh chodesh combines with that of shabbos to create a new, synthesis kedushas hayom (harkavah mizgit). The korban musaf of this shabbos has the benefits of both kedushos hayom together and is no proof to an ordinary tadir vs. mekudash issue.
This is clearly against what I wrote in the previous post, but R' Yosef Engel has much more to say on the issue.