In my limited experience I haven't come across "idir" being used for "both". I used "araon" but was marked incorrect.
Judging by example usages on Potafocal, Tá cairéid agus fataí araon san anraith should be accepted.
Idir … agus seems to be used only for grouping disparate nouns, e.g. Tá idir fhear agus bhean anseo, while araon seems to be of more general use, e.g. Tá siad araon anseo.