فَبِأَيِّ آلَاءِ رَبِّكُمَا تُكَذِّبَانِ
Which of your Lord’s favours will you both deny?
Both kumā in rabbikumā and the verb tukadhdhibān are dual in form. In the surah’s broader context the address is widely understood as directed to humans and jinn. A singular or plural rendering without noting the dual would conceal a formal feature.
The dual morphology is a grammatical observation; identifying the two addressed groups is established by wider context and exegetical reading. Keep these levels of explanation distinct.