adverb
back to back
Describes two things or people positioned with their backs facing each other.
二人は後ろ合わせに座った。
The two sat back to back.
本棚を後ろ合わせに並べる。
Arrange the bookshelves back to back.
Standard kanji spelling.
Variant with 合せ instead of 合わせ.
背中合わせ is more common and means 'back to back' with a nuance of mutual reliance or opposition, while 後ろ合わせ is a simpler, less common term for physical back-to-back positioning.
Compound of 後ろ (ushiro, 'back') and 合わせ (awase, 'joining, matching'), literally 'back-matching'.