May one Daven Mincha and Maariv within Plag Hamincha on Erev Shabbos or during the week?
It is forbidden to Daven both Mincha and Maariv on a single day within the period of Plag Hamincha. [Some Poskim however are lenient to allow a congregation to Daven both prayers one after the other within the period of Plag Hamincha, if it will be difficult to gather the congregation again afterwards for Maariv. Other Poskim however rule one may only be lenient to do so if one will Daven Maariv after sunset. There are communities that follow the former opinion and Daven both Mincha and Maariv within Plag Hamincha. It is never permitted for an individual that is not part of a Minyan to Daven both Mincha and Maariv within Plag Hamincha, as stated above.]
 Admur 267/2; M”A 267/1; M”B 267/3; Biur Halacha 267/2 “Umiplag”; Piskeiy Teshuvos 267/2 footnote 19
The reason: As this is a contradiction in regards to whether one holds the period of Plag as day [Mincha] or night [Maariv]. [ibid]
 Regarding weekdays: M”A 233/6; M”B 233/11; Poskim in Kaf Hachaim 233/12; Regarding Erev Shabbos: Derech Hachaim Arvis 1, brought in M”B 267/3; Biur Halacha ibid; Shraga Hameir 5/21; Kinyan Torah 4/27; See Piskeiy Teshuvos 233/3; 267/2
 M”B 267/3; Biur Halacha ibid regarding Erev Shabbos; Vetzaruch Iyun from M”B 233/11 which records the ruling of the M”A without argument.
 See Shraga Hameir 5/21; Kinyan Torah 4/27