According to Mishnah Berura 233:11, it sounds like if the only available minyan was "tartei desasrei," i.e. mincha after plag and maariv immediately after, it would still be preferable than davening alone.
However, in Mishnah Berura 267:3 and Biur Halacha "uviflag," it seems to follow that davening "tartei desasrei" is never a valid option.
Does anyone know what would be considered better?