If by "Kabalat Shabbat" you mean Ma'ariv (according to the Shulchan Aruch when the Chazan says Barchu at Ma'ariv, the congregation is Mekabel Shabbos; see Mishnah Berurah 267:1), then you are right - a congregation shouldn't daven both Minchah and Kabalat Shabbat within Plag ha'Minchah (the Mishnah Berurah, 267:3, brings an opinion that is lenient in this matter, but says that it shouldn't be relied upon). However, if you mean the Kabalas Shabbos that we are accustomed to saying before Ma'ariv, then there is no problem saying both of them within Plag ha'Minchah and, in fact, the Minhag in many places is to daven Minchah twenty minutes or so before sunset and then start Kabalas Shabbos immediately after (usually a few minutes before the Shkia).

Kol Tuv,

Yonasan Sigler