Its actually said in the name of Gdolim like Reb Shlomo Zalman (Ginzei Hakodesh 3 30, I haven't seen it) that you shouldn't put Torah Shebaal Peh on top of a Siddur (different from the Aruch Hashulchan), and its Kdusha is like Neviim and Ksuvim since it has psukim from Tanach in it.
It is like Neviim and Ksuvim because in YD 282:19 (from Megillah 27a), The Rama says the dinim of not putting Neviim and Ksuvim on a Chumash and a Chumash on a Sefer Torah is only with two Krichos, but in the same one Hakol Shari.
So the svoro might be its the same as with tfillos and tanach printed together.
Its not like a "Chumash" even though it has Torah in it because there's a difference between gilyon and kricha (Rama YD 283:1)
But they don't need to rely on the Aruch Hashulchan because they are possibly differentiating between printing together and placing one on another. Just like we see by putting a Sefer Torah on Neviim that you can't but you can write them together.
Who are the Poskim who hold a Gemara goes on top of Nach and what is the svoro? The Ri Migash for one says it can't and those mentioned before. שו"ת הר"י מגאש סי' צב
On the ball,
even from the Aruch Hashulchan the Svoro goes further than the chshash of mixing up the Kdusha, rather, with their actual Kdusha. In fact, the Zohar says "תורה שבכתב ישוון יתה על תורה שבעל פה", not vice versa.
(פ' נח, ח"א עא,ב)
the Rama (YD 282,19) says we are not makpid about the placement of ksuvim (divrei kabala,nevuah) on neviim (divrei kodesh, ruach hakodesh), even though they have different kdushos (Ran (Rif Megilla 8,2).