According to my Artscroll Chumash, it appears that parshat Ki Tisa can not be the scheduled Shabbat reading on Shabbat Shekalim (I assume this because the potential maftir and haftarah readings listed on page 515 include only the regular reading and the reading in the case of Parshat Parah). I assume that this is a certainty now that we have a fixed calendar. But I wonder if it was ever possible for Ki Tisa to coincide with Shabbat Shekalim.
If so, would there be a maftir for Shekalim (which is normally from the beginning of Ki Tisa) or would we have discharged our Shekalim obligation through the regular Torah reading of the exact same portion?