It is forbidden to us another siddur to keep the one you are using open.
Explanation: Of course, you are not allowed to use or do to Sifrei Kodesh anything degrading.(Shulchan Aruch YOD 282).For example you may not throw Sifrei Kodesh Cv”S even midrashim. The Rema say one may not even place them on his lap or lean on it…
The question here is this considered a degrading thing to do to a Sefer.
There are contradictory statements in the Sefer Chassidim. In Siman 502 he writes that one should not put a sefer on top if the line he is copying even though he does not want to make a mistake. In 504 he writes if the sun is shining on the sefer he is learning and he can’t see he may put a seer to block the light.
The Bircat Naftali answers it is forbidden to use like a bookmark for which lien he is writing since it is possible to copy without it there it just to make it easier. However, when the sun is shining and he eh can’t see the page, he may use another sefer to help him learn. R’ BZ Abba Shaul and R’ Elyashiv both say similar ideas.
The Sefer Avnei Yashpeh answers that if you can use something else easily but you use a sefer instead it is degrading, but if there is nothing else to use it is not degrading for it to help him learn.
R’ SZ Auerbach answered simply it depends if it seems like a degradation what he is doing or not.
In regards to placing one sefer on top the other to lift up the seer the you are using. The Taz (YOD 282:13) writes: it seems to me that those people who when they are learning who take another sefer and place it underneath to raise it up is forbidden because it’s a degradation. The Magen Avraham (154:14) argues with Taz and says it is permitted. So rules the Chaye Adam and the Mishna Berura. The Piskei Teshuvot extrapolates from her that is permitted to use a sefer for anything which is for learning and it is is not degrading. Even using it as a bookmark then would be permitted.
The Yalkut Yosef though rules like the Taz and forbids using a sefer to prop up another sefer to learn from it. It seems the only thing permitted to do with sefarim are things which aren’t degrading.
This is all a discussion about using another sefer to help you learn however for Tefilla it might be different. It would seem to be forbidden to use another siddur to keep the one you are using open. More stringent then by learning. However, if you really can’t pray the sefer Shmirat Kavod Hasefarim writes that should not put your Tefillin cover on the siddur to keep it open unless you can’t pray since the pages are closing then you are allowed to put it there. So too a siddur.
