﴿ مُّتَّكِـِٔينَ عَلَيْهَا مُتَقَٰبِلِينَ ﴾

“reclining upon them, facing one another [in love].”

reclining on them face to face muttaki’īna ‘alayhā mutaqābilīna constitute two circumstantial qualifiers referring to the subject person of the predicate ‘they’.