The Laws of Prayer (The morning services) (51)

1. One who has finished the entire Amidah and has also said the verse“Yiheyu leratzon etc.”, even if he has not yet said Elokai Netzor, may answer Kaddish, Kedushah and Barechu. (Shulchan Aruch and Mishne Berurah 104)

2. One who is praying two consecutive Amidah prayers (e.g. the morning and additional services, or he made a mistake for which he has to repeat the entire Amidah), must pause between them for the amount of time that it takes to walk four cubits so that his mind is focused to be able to pray as a supplication. (ibid)