is in apposition, roughly, “you who know [these things] well” – with the emphasis being a kind of flatter, “if this seems so to you, you who know these things well”.
the ἡμῖν in the second half of the sentence is related to ἀνάγκη. ἀνάγκη ἡμῖν [ἐστίν]. τί ἀνάγκη ἐστίν? συγχωρεῖν.