Totality of effect, not of cause: the two horses
Thesis LXV. They grant that the whole supernatural act depends on grace, but add that it depends on grace not totally but only in part, because grace cannot produce it except in dependence on the will's concurrence.
Grace can therefore be called the total cause of the good act by totality of effect, but not by totality of cause, which is the point at issue here. The whole effect does depend on grace, but as on a cause that does not produce it without another cause acting with it at the same time.
That is why they compare grace and free choice to two horses pulling the same cart together. Neither of them acts first and moves the other to act, and each is only a partial cause of the pulling, although the whole pulling can be said to depend on each horse, since each contributes something to all of it.
Latin
LXV. Admittunt quidem totum actum supernaturalem a gratia pendere: verum addunt illum non pendere ab ea totaliter, sed partialiter tantum: quia gratia, scilicet, nequit illum producere, nisi dependenter a concursu voluntatis. Ac proinde gratiam posse quidem dici causam totalem actus boni totalitate effectus, sed non totalitate causae, de qua hic agitur: quoniam totus quidem effectus a gratia pendet, sed tamen ut a causa quae illum non producit sine alia causa concurrente, & simul cum ea agente. Unde est quod comparant gratiam & liberum arbitrium duobus equis currum eundem simul trahentibus, quorum neuter altero prius agit, & alterum movet ad agendum, & qui tractionis illius sunt causae tantum partiales; licet tota tractio dici possit, ab equis singulis dependere, quia singuli ad totam tractionem aliquid conferunt.