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Stand facing each other.
Literal
Face-to-face-oppose-mutually stand-[command-なさい].
向かい合う ('to face each other') is the reciprocal compound verb built from 向かう ('face, head toward') + 合う ('mutually'). ~合う is a productive suffix for mutual/reciprocal actions: 話し合う ('discuss each other'), 愛し合う ('love each other'), 助け合う ('help each other'). Te-form 向かい合って plus 立つ ('stand') gives 'stand facing each other.'