The Shulhan Aruch (Orah Haim 144) writes that a congregation must ensure before the Sefer Torah is brought from the ark for the reading that it is set to the right place in the Torah. If the Torah must be rolled to the right place after it is taken out of the ark, this inconveniences the congregation, and such a situation must be avoided. If the congregation realizes during the Pesukeh De’zimra section of the prayer service that the Sefer Torah had not been prepared and rolled to the proper place, then somebody must go and roll the Sefer Torah at that point so it is prepared by the time it is taken from the ark. Similarly, the Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933) comments that if, on a Shabbat when two Sifreh Torah are read, it is discovered that one of the two scrolls is not set to the right place, it should be rolled while the first Torah is being read, in order to not to inconvenience the congregation.
On a day when two Sifreh Torah are read, such as Shabbat Rosh Hodesh, a person may not be called for Aliyot to both Sifreh Torah. Meaning, if a person had been called for one of the seven main Aliyot, he may not then be called for the Maftir reading from the second Sefer Torah. The reason is that people might suspect that he was called for a second Aliya because the first Sefer Torah was found to be disqualified. In order that people will not cast aspersions on the first Sefer Torah, a Mesader should not call for Maftir somebody who had already received an Aliya to the first Sefer Torah. This applies also in a case where three Sifreh Torah are read. Meaning, if a person had received an Aliya to the first Sefer Torah, he may not be called to the third Sefer Torah for Maftir, despite the fact that a different Sefer Torah had been read in the interim.
Under certain circumstances, the Halachic authorities permit calling a person for two Aliyot to the same Sefer Torah. Namely, if there is only one Kohen or Levi present in the synagogue, and he must therefore receive the first Aliya (in the case of a Kohen) or the second Aliya (in the case of a Levi), he may receive also the Aliya of Maftir for a Yartzheit. Since there is no other Kohen or Levi, he must receive the first or second Aliya, and he may then also receive Maftir for the Yartzheit, provided that the Maftir is read from the same Sefer Torah. Interestingly enough, this Halacha can result in a situation where a person receives three Aliyot on the same Shabbat – if the only Kohen in the synagogue has a Yahrtzeit, and there is no Levi present in the synagogue. When no Levi is present, the Kohen who had received the first Aliya is called again for the second Aliya, in place of a Levi. If he has a Yartzheit that day, he may also receive the Aliya of Maftir, as long as the Maftir is read from the same Sefer Torah as the previous Aliyot.
Summary: Congregations must ensure that the Sefer Torah is rolled to the proper place before it is taken out of the ark, so the people will not have to wait for the Torah to be rolled before the reading. As a general rule, a person should not receive two Aliyot during the same reading. If, however, there is only one Kohen (or Levi) in the synagogue, and he must therefore receive the first Aliya (or second, in the case of a Levi), and he has Yahrtzeit that Shabbat, he may receive Maftir for the Yahrtzeit. If, however, a different Sefer Torah is read for the Maftir, such as on Shabbat Rosh Hodesh, then one who received an Aliya to the first Sefer Torah may not receive Maftir under any circumstances.