105:1. One who prays (1) two Amidos, one after the other, must (2) wait (3) between one and the other (4) long enough to walk four amos (paces), in order that his mind be settled in order to pray as a request.
MB 1: Two Amidos – For example, the Morning Prayer and Additional Service [on Shabbos, Rosh Chodesh, or a Holiday], or one who forgot to pray previously and must now make up for it during the following prayer time-period [one must make up for the Morning Prayer during the time for the Afternoon Prayer, etc.] and thus must pray twice. Note that according to what we will explain in Siman 108 in the Mishna Brura, namely that whether one forgot the Evening Service and must pray twice in the morning, or whether one forgot the Morning Service and must pray twice during the afternoon to compensate, one must say Psalm 145 (“Ashrei”) in between, if so then this Law can only apply when one forgot the Afternoon Service and thus must pray twice during the evening for compensation, because then one does not say Psalm 145 as will be explained there in the Mishna Brura – meaning that nonetheless one must wait at least long enough to walk four amos.
MB 3: Between one and the other – In the book Shulchan Shlomo, it says that so too if one completed the Eighteen Blessings, even though he has not yet moved back, and recalled that he made a mistake and must go back, then he should wait.
MB 4: Long enough to walk four amos – Even if one does not want to step forward again, but rather intends to pray the second time in the place where he is now standing after backing away from his first prayer; [because] in order to return to one’s place one must wait even if he does not intend to pray a second time, as we will see in Siman 123.