Question: Is it an option to switch readers in the middle of one aliyah such that no berakhot are said between readers? Or does it have to become a 2nd aliyah? This question is particularly important as our minyan would like to honor the tradition of reading the entire story of עגל הזהב as part of the second aliyah reserved for a Levi, but we have no one skilled enough to read the entire first aliyah.
Answer: The practice of prolonging the first aliyah of Ki Tissa in order to have the entire story of חטא העגל contained within the second aliyah is mentioned in Magen Avraham 428:8 (R. Avraham Gombiner, Poland, 17th c.). The practice seemingly reflects the fact that Levi was the only tribe that did not participate in this sin and it would thus be shameful for any other Jew to receive the aliyah about this story. However, in the case you describe, there is no problem with having a second Torah reader come up in the middle and take over; see Responsum Har Tzvi OH 1:72 (R. Tzvi Pesah Frank, 20th c., Israel) for one explicit statement to this effect. This has no effect on the aliyah numbering, which is determined solely by the number of sets of berakhot that are said by olim. It is prefereable for the second torah reader (and any others reading parts of that aliyah) to be there for the beginning of the aliyah to hear the opening berakhah such that all readers are a part of this unit in its entirety.
I will note that the one hesitation one should have with this system is the possibility that it will lower standards for Torah reading and that it may dampen individual and communal enthusiasm for cultivating expert readers who can show true כבוד התורה through their investment in this important and public reading of our most sacred text. I will just quote Rosh Megillah 3:1 (R. Asher b. Yehiel, Germany/Spain, 13th-14th c.) to this effect. He argued that one should not have a professional Torah reader and that people of inadequate skill should indeed feel somewhat inadequate and cede the floor to those more skilled, such that they be inspired to work towards a greater skill level. While there is an important role for encouraging participation, splitting aliyot in this way should be done sparingly so as not to deprive people of the chance to stretch themselves to improve their Torah reading skills.
Written by Steven Exler and Rabbi Ethan Tucker