The rule that a Bracha may not refer to two themes is presented as follows:
תלמוד בבלי מסכת ברכות דף מט עמוד א-תנו רבנן: מהו חותם? בבנין ירושלים; רבי יוסי ברבי יהודה אומר: מושיע ישראל. מושיע ישראל אין, בנין ירושלים לא? - אלא אימא: אף מושיע ישראל. רבה בר רב הונא איקלע לבי ריש גלותא, פתח בחדא וסיים בתרתי. אמר רב חסדא: גבורתא למחתם בתרתי! והתניא, רבי אומר: אין חותמין בשתים! גופא, רבי אומר: אין חותמין בשתים.
Translation: Our Rabbis taught: How does one conclude the Bracha of the building of Jerusalem in Birkat Hamazone? R. Jose son of R. Judah says: Saviour of Israel. ‘Saviour of Israel’ and not ‘Builder of Jerusalem’? Say rather, ‘Saviour of Israel’ also. Rabbah b. Bar Hanah was once at the house of the Exilarch. He mentioned one theme at the beginning of the third blessing and both at the end. R. Hisda said: Is it better to conclude with a Bracha that presents two themes? Has it not been taught: Rabbi says that we do not conclude with a Bracha that presents two themes.
This rule leads to answering the question: how did Jewish Liturgy end up with so many Nusacha’Ot? Today the primary Nusacha’Ot are the two Ashkenazic Nusacha’Ot, Minhag Polin and Minhag Ashkenaz, Eidat Ha’Mizrach which is a term that does not do justice to the Sephardic community because of variations such as Morocco and the Spanish Portuguese Nuscha’Ot, Yemenite, Nusach Roma and Nusach Sfard / AR”I. When you add the extinct Nusacha’Ot like Romania, Aram Tzova, Kappa and Carpentras, you recognize that there must have a been a beginning to this division. I would like to suggest that the split began with the two major Jewish communities that represented Judaism during the period of the Geonim. Thanks to the uncovering of the fragments from the Cairo Geniza, we can suggest that the division began with two co-existing Nusacha'Ot Minhag Bavel (Babylonia) and Minhag Eretz Yisroel. That two Nusacha’Ot developed during the same time period should not be surprising since two versions of the Talmud developed as well.
Brachot Rule number 6 is early evidence that a divide between Minhag Bavel and Minhag Eretz Yisroel developed.
I have already established that the most important element of a Bracha is its חתימה because the purpose of the חתימה is to summarize the theme of the Bracha. That is why including two themes in a Bracha can be problematic. Nevertheless, the rule was not initially followed in Minhag Eretz Yisroel. Here are two examples from versions of Shemona Esrei that Professor Uri Ehrlich presented in his book תפילת העמידה של ימות החול: נוסחי הסידורים בגניזה הקהירית. First the Bracha of בונה ירושלים:

Now the Bracha of שים שלום / שלום רב:

The differences in these two Brachot are significant because they provide the background to some elements of current liturgical practices. Let me focus first on the Bracha of בונה ירושלים. Why in Minhag Eretz Yisroel did they refer to both אלקי דוד and בונה ירושלים in the חתימה? It is because in Minhag Eretz Yisroel, they did not recite the Bracha of את צמח דוד. They maintained the rule that Shemona Esrei was to consist of only 18 Brachot. It was Minhag Bavel that split the Bracha into two. That is why we now know that the 19th Bracha was not ברכת המינים but was את צמח דוד. Why did they split the Bracha? Because the original Bracha had two themes and both themes were reflected in the חתימה. Are you surprised that ברכת המינים was not the 19th Bracha? The following excerpt from the Tosephta confirms that ברכת המינים was one of the Brachot of Shemona Esrei when Shemona Esrei consisted of only 18 Brachot:

Translation: The number 18 Brachot was established by the Sages representing the 18 times that G-d’s name appears in the Psalm that begins: Havu L’Ashem Bnai Ailim. It is acceptable for one to include the message of the Bracha concerning the apostates with the Bracha concerning the Perushim (Al Hatzadikim) or the Bracha concerning the Garim with the Bracha concerning the elders or the Bracha concerning the Kingdom of David (Et Tzemach) with the Bracha of Boneh Yerushalayim; if one separated these themes into separate Brachot, he fulfilled his obligation.
One more point about Nusach Ashkenaz. I have already presented the rule that the theme of the Bracha must be reflected in the words that precede the concluding formula. Did you ever notice the words that are said before the חתימה of the Bracha of בונה ירושלים in Ashkenazic Nusacha’Ot?
וְכִסֵּא דָוִד מְהֵרָה לְתוֹכָהּ תָּכִין.
That is the theme of את צמח דוד. The Ashkenazic Nuscha’Ot left the wording in to remind their adherents that the two Brachot were once one. Other Nusacha’Ot either omit the words or recite the words in the middle of the Bracha. In Nusach Roma the words are omitted:

In Sephardic Nusacha’Ot the words appear in the middle of the Bracha:

A second point concerning the Ashkenazic Nusacha’Ot. A question has arisen in the last few decades as to whether to change the חתימה of שים שלום / שלום רב during עשרת ימי תשובה. What we learn from the Cairo Geniza is the words עושה השלום were always included in the חתימה in Minhag Eretz Yisroel and that over time Minhag Eretz Yisroel adopted the rule that the חתימה should not include two themes and modified the Bracha to just עושה השלום. What that means is that the מטבע הברכה for the last Bracha of Shemona Esrei differed whether you followed Minhag Bavel or Minhag Eretz Yisroel. Those who followed Minhag Bavel, which was the practice for the Sephardic Nusacha’Ot, the original מטבע הברכה was המברך את עמו ישראל בשלום. Thus, it should not be changed during the עשרת ימי תשובה. However, the Ashkenazic Nusacha’Ot were closely linked to Minhag Eretz Yisroel. Two quick examples: the Ashkenazic Nusacha’Ot interrupted the ברכות of קריאת שמע and שמונה עשרה with Piyuttim, a practice criticized by the Geonim of Babylonia and a practice not followed by the Sephardic Nusacha’Ot. Second, opening Kedushah with כתר יתנו was not adopted by most Ashkenazic Nusacha’Ot. The only exception was מנהג צרפת, the French Ashkenazic Nusach which opened Kedushah for Mussaf on Shabbat and Yom Tov with כתר יתנו. One additional way in which the Ashkenazic Nusacha’Ot demonstrated their allegiance to Minhag Ertez Yisroel was to adopt the Minhag Eretz Yisroel חתימה of עושה השלום during the עשרת ימי תשובה. They also adopted the Minhag Eretz Yisroel חתימה for the Bracha of מודים when their Kohanim performed ברכת כהנים on holidays.
