If on Erev Shabbos one placed a pot on top of another pot, may he later on Shabbos place the upper pot on the blech or electric plate?
According to all, if when one removed the upper pot from the flame and placed it onto the lower pot on Erev Shabbos he had fulfilled all the Chazara conditions [intention, did not let go in between] then he may place it directly on the blech on Shabbos. If the conditions of Chazara were not fulfilled, then there is a dispute amongst Poskim as to the ruling. Practically in a time of need for Oneg Shabbos one may be lenient to place it on the blech. Although it is best to place the pot on top of an upside-down pot or plate, rather than directly on the blech or electric plate.
 Shabbos Kehalacha Vol. 1 pages 418
 The Sheivet Haleivi and Shemiras Shabbos Kihilchasa both rule that it is forbidden to do so as the upper pot is not considered to have been on a fire when Shabbos began and thus it is considered as if he is initially placing it on the fire on Shabbos. However other Poskim allow this.