From SA"OC #235 it appears initially one should not do so, and that only a minyan can daven Ma`ariv early so as to not encumber them to assemble again if they had just done a late Minchah (i.e. after Pelag.).
My rav didn't have time to elaborate when I asked him but only said:
No problem doing both after plag.
I looked up in Yalkuth Yossef and he writes concerning this:
...ve gam bazeh kevar nahagu lehaqel, veyesh lahem `al mah sheyismokhu.
Translation: But even this, the custom has already been to be lenient; and they have what to rely on.
There is a long footnote there from Yechaweh Da`ath; seems to me the following is the essence:
...ולדעת חכמים זמן תפלת מנחה עד הערב, ולא הוצרכו בגמרא להשמיענו אלא שיכול גם
כן להתפלל ערבית מפלג המנחה כרבי יהודה
Translation: And the consensus of the Sages is: Mincha time is until evening [whereas others said it is only earlier], so the Gemara is only citing this to tell us one can also daven Ma`ariv from Pelag haMinchah onwards like Rabi Yehudah.