Question

Rabbi Eliezer Melamed writes: On those Sabbaths when these portions are read, two Torah scrolls are taken out. From the first, seven people are called up for the weekly Torah portion, and from the second the maftir reads the special portion. And since the haftarah from the Prophets should deal with what was read from the scroll to which the maftir was called up, therefore on those Sabbaths the haftarah deals with the matter connected to those portions that were read from the second scroll, and not with the weekly portion that was read from the first scroll.
According to my understanding of what he wrote, the reason they take out a second scroll is because the haftarah that is read needs to belong to the portion that was read from the Torah scroll. So a problem arises when only one Torah scroll is taken out, because they read from it both the weekly portion and the special portion. But my study partner understood it differently, and that is also how the rabbi there explained it: that the reason they take out a second scroll is because of burdening the congregation, and Rabbi Melamed is only describing the chain of events here or something like that. How does the Rabbi understand his words, and what is the actual ruling?