hi,
(a) πάντα τὰ ὑπά?χοντα is the subject of συνᾴδει (nominat., not acc.), and
(b) τἀληθές is the subject of διαφωνεῖ (nominat. sg., = τὸ ἀληθές).
you may want to read the start of Aristotle’s ΠΕΡΙ ΕΡΜΗ?ΕΙΑΣ which introduces the vocab of statements, truth &c.
cheers 
To explain further, I believe the datives are of reference.
τῷ ἀληθεῖ “in/with a true (statement)”
τῷ ψευδεῖ “in/with a false (statement)”
The adjectives true and false are being used as substantives, which is common.
To rephrase the original sentence:
πάντα τὰ ὑπά?χοντα συνᾴδει τῷ ἀληθεῖ
[Every fact harmonizes with the true (statment)]
τἀληθές διαφωνεῖ τῷ ψευδεῖ
[the true (statement) discords with the false (statement)]
Also, notice the construction of μὲν… δὲ, which set’s up the two halves of comparison “on one hand… on the other.”
Thanks!
The only thing I’m still a bit confused about is, how can πάντα τὰ ὑπά?χοντα be the subject of συνᾴδει? Isn’t πάντα τὰ ὑπά?χοντα neuter plural, while συνᾴδει is singular?
hi, see smyth s958, which answers your query:
http://www.perseus.tufts.edu/cgi-bin/ptext?doc=Perseus%3Atext%3A1999.04.0007&query=head%3D%23301
cheers
Oh, wow, that’s cool. Thanks for the help!