It is a conditional sentence: first an if-clause (“protasis”), then the main clause (“apodosis”). For conditionals see the first post in http://discourse.textkit.com/t/conditionals-a-guide/14120/1
The if-clause is εαν … ποιησώμεθα: “If we make our agreements …”
(εαν+subj. “if we do something”)
The main clause is οὕτω … συμμένοιμεν ἄν, “in this way we would abide by them.”
(opt.+αν “we would do something”)
In Smyth’s terms, the protasis is “future more vivid,” while the apodosis is “future less vivid” (Smyth 2297). So this is not “if A happens, B will happen” (fut.indic. in main clause) but “if A happens, B would happen.” He shies away from saying that they will abide by their agreements and settles for saying that in those circumstances they probably would.
ᾗ and ταύτῃ are correlative, the ταυτῃ picking up the relative ᾗ: ἐὰν οὖν ᾗ ἑκατέροις μάλιστα συνοίσει, ταύτῃ καὶ τὰς συνθήκας ποιησώμεθα, lit. “(So if) in what way it will most benefit each side, in this way we also make our agreements,” “if we frame our agreements in the way in which it will be of most benefit to each side”.
You correctly split the sentence into three clauses, but the ᾗ clause (a relative clause) is part of the if-clause; note how the ᾗ follows the ἐὰν, embedding the ᾗ clause within the ἐὰν clause.
οὕτω κατά γε τὸ εἰκὸς μάλιστα συμμένοιμεν ἄν: “in this way (οὕτω) in all probability (κατά γε τὸ εἰκὸς) we’d be most likely to abide by them”
So the sentence is “So if we frame our agreements in such a way as will be of maximum mutual benefit we’d maximize the chances of our abiding by them.”
κατά γε τὸ εἰκὸς: the γε qualifies the phrase: it’s likely, but no more than that.