The Torah reading of Parshas Ki Seitzei:
If one missed Parshas Zachar: If one did not hear the reading of Amaleik on Parshas Zachar he can still fulfill his Biblical obligation through hearing the Torah reading of Amaleik in Parshas Ki Seitzei. [In such a case however one must have in mind to fulfill the Mitzvah of remembering Amaleik upon hearing the reading. Some Poskim write that one is to tell the Baal Korei to have in mind to fulfill his obligation.]
Zeicher/Zecher: There is dispute amongst Poskim as whether one is to read Zeicher Amaleik or Zecher Amaleik. Practically one is to read both. By the reading of Parshas Zachar and Parshas Ki Seitzei one reads first Zeicher and then Zecher. By the reading on Parshas Beshalach and Purim one reads first Zecher and then Zeicher. Our custom is to only repeat the word Zeicher and Zecher, however others have the custom to repeat the entire verse twice. [One is to repeat Zeicher Zecher in both Shevii and the Haftorah.]
Haftorah of Shabbos Ki Seitzei:
If Rosh Chodesh Elul falls on Shabbos then one reads the Haftorah of Shabbos Rosh Chodesh “Hashamayim Kisi”. [In such a case, on Shabbos Parshas Ki Seitzei the regular Haftorah is followed by the Haftorah of Ania So’arah.]
 Aruch Hashulchan 685/5; Hisvadyos 1989 2 p. 442-446 printed in Shaar Hamoadim Adar 46-47
 Har Tzevi 1/58
 Shaareiy Halacha Uminhag 2/288; Ketzos Hashulchan 84 footnote 22 and glosses to volume 3 (p.74b)
 Brought in M”B 685/18 [The Rebbe in Shaareiy Halacha Uminhag ibid writes his source is from the Mesorah] Some Poskim rule the main Nusach is with a Tzeirei. [Betzeil Hachachmah 6/50; Piskeiy Teshuvos 685/9] Others rule the main Nusach is a Segal. [Maaseh Rav 134 that so was the custom of the Gra; Ketzos Hashulchan 84 footnote 22. In the glosses to volume 3 (p.74b) he proves that this was also the opinion of the Tzemach Tzedek] In conclusion the Rebbe and Ketzos Hashulchan ibid rule that by Beshalach the main Nusach is with a Tzeirei while by Ki Seitzei the main Nussach is with a Segal. Nevertheless we still read both Nuschaos as will be explained.
The saying of the Toras Chesed: The Ketzos Hashulchan ibid quotes the Toras Chesed to say “Zecher, Zeicher, the main thing is to blot them out properly”.
 M”B ibid; Ketzos Hashulchan ibid writes that those which are meticulous would read it one time with a Tzeirei and one time with a Segal. Meaning that by the reading of Beshalach and Ki Seitzei they would read it one way by Shevi and the second way by Maftir, while by Purim and Parshas Zachar they would repeat the verse twice. In the glosses to volume 3 (p.74b) he concludes that this is the custom, to read both Zeicher and Zecher and so was the directive of Rav Shneur Zalman of Lublin, the author of Toras Chesed. To note however that there he mentions that the Baal Korei in Lubavitch stated they would read by Beshalach only with a Tzeirei, and by Ki Seitzei only with a Segal.
 So concludes the Rebbe ibid, Ketzos Hashulchan ibid based on the Sefer Boneh Yerushalayim which is Meyuchas to the Alter Rebbe that by Beshalach the main Nusach is with a Tzeirei while by Ki Seitzei the main Nussach is with a Segal. Nevertheless we still read both Nuschaos as ruled the Toras Chesed.
 See Piskeiy Teshuvos 685/9
 Otzer Minhagei Chabad p. 18 unlike the custom recorded in Ketzos Hashulchan 85 footnote 22 and the supplements in the back of the Sefer that one is to read Zeicher by Shevi and Zecher by Maftir.
 Rama 425/1 in name of Tur and Mordechai; Mateh Efraim 581/8; Kitzur SHU”A 128/4; Sefer Haminhagim p. 64 [English]
Background and other opinions:
Throughout the year when Rosh Chodesh falls on Shabbos the Haftorah that is read is Hashamayim Kisi. [Michaber 425/1] However regarding Shabbos Rosh Chodesh Elul the Michaber ibid rules one is to read the Haftorah of Aniyah Soeira while the Rama ibid rules that one is to read the Haftorah of Rosh Chodesh as is usually done.
 Ketzos Hashulchan 88 footnote 15 “There is no need to add two verses of Ania Soeira at the end of the Haftorah”
 Mateh Efraim ibid; Kitzur SHU”A ibid; Sefer Haminhagim ibid