1. One who prays a supplementary prayer, should bring it as close as possible to the obligatory prayer that he is praying. He should not interrupt between them for anything even learning Torah. It is therefore obvious that he may not eat between the prayers before he has performed the supplementary prayer. Some even say that if he started to eat he must stop. (Shulchan Aruch and Mishne Berurah 108)
2. However, matters related to praying itself are not considered an interruption, and it follows that when praying a supplementary prayer for the evening prayer during the morning prayer, one should wait after the first (obligatory) Amidah and listen to the repetition of the Amidah by the cantor, recite “Tachanun” and “Ashrei” and only then say the supplementary prayer followed by “Lamnatzeiach” and “Uva Letzion”. When supplementing the morning services during the afternoon services, one should say “Ashrei” in between them. When supplementing the afternoon services during the evening services, some say that he should say “Ashrei” between them, and others maintain that he should not. (ibid)
