One is permitted to place a Tehillim on top of a siddur that contains all of Tehillim, and likewise to place such a siddur on top of a Tehillim.
Sources and Explanations
Rivevot Ephraim (vol. 1, siman 119, ot 2) cites opinions on this matter. He initially writes that the sanctity of a siddur that contains Tehillim is greater than that of a book of Tehillim on its own, since the siddur contains portions of the Torah and the Prophets; therefore it is preferable to place the siddur with the Tehillim above the book of Tehillim. However, he cites that HaGaon Rabbi Shmuel Wosner zt”l was asked about this, and he replied that there is no concern, and one may also place the Tehillim on the siddur.
From the sefer “Shaf Veyativ – Laws of Saying Tehillim” by Rabbi Shay Tahan, Chapter 8: The Sanctity of the Sefer Tehillim and Its Honor, Halacha 1 (p. 133 in the print edition). Translated from the Hebrew original.
