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I would say this is tmesis (prefix of a compound verb separated from main part of the verb), or else an adjectival use of σύν, in either case with a sense of "together, in conjunction." --> he seized two men at once
One big clue that it's not functioning as a normal preposition is that you don't have a noun in the dative here.
IPHIGENIE: Kann uns zum Vaterland die Fremde werden?
ARKAS: Und dir ist fremd das Vaterland geworden.
IPHIGENIE: Das ist's, warum mein blutend Herz nicht heilt.
(Goethe, Iphigenie auf Tauris)
What is the function of ὅ here? Is it the masc. nom. sg. article as subject of verb and aor. part.? I'd assume so. If it's supposed to be neut. rel. pron. with δόρπον as antecedent, the order in which I should read the sentence seems convoluted.