I have seen the minhag in certain shuls that on shabbos the second sefer torah isn’t held during the reading but rather is placed on the bima in a certain way that it is held upright and then chained around the sefer torah can’t fall. I recall in the back of my mind a teshuva in the Iggros Moshe that disagrees with this custom, is there a support to this minhag?
Rema (147:8) writes that the second sefer Torah should be held by somebody until it is read, and Shaar Efraim (10:6) adds that it should be given to a ben daas, and not to a youth (see also Mishnah Berurah 147:29). In the shul of the Shach, the sefer Torah was placed in a special device that held it until it was needed for reading. Minchas Yitzchak (2:117) permits using this device (see also Daas Sofer 1:20), but, as you correctly note, Iggros Moshe (1:38) prohibits its use, and writes that it is preferable to leave the sefer Torah in the Aron, rather than take it out without somebody holding it.
Thus, there is some support for the custom, yet those who follow the rulings of Rav Moshe should ensure that somebody actually holds the sefer Torah.