فَعَلَيْهِمَا جَزَاءٌ وَاحِدٌ
Translation: Thus one compensation is due upon them both.
Fa introduces the consequence.
ʿAlayhima is a prepositional phrase containing the dual pronoun “them both,” forming the fronted predicate.
Jazaʾun is the delayed nominative subject.
Wahidun is its nominative adjective.
“One” describes the compensation, not just one of the two participants.