後うしろ合あわせ

adverb

back to back

Describes two things or people positioned with their backs facing each other.

• 二人ふたりは後うしろ合あわせに座すわった。

  The two sat back to back.

• 本ほん棚だなを後うしろ合あわせに並ならべる。

  Arrange the bookshelves back to back.