In this picture we see a strike to the chest with a pass, in falso from down to up, between the weapons. You need to press the weapons together well for this to work.
The opponent is in terza with both weapons extended on an oblique line, so that the point of his sword is aimed at your right shoulder, and the point of his dagger to your left shoulder.
Also take a terza, with the point of your sword low, but your dagger on a high line, bending your body to the left.
When he approaches (it does not matter much why he does this, it'll work in any case), on the tempo of his advance pass with your left foot toward his right side, doing one of the following:
1) parry with the dagger over your right arm, and thrust him from down to up between his weapons, or
2) disengage over his sword, and press his sword with both weapons (shown), and strike him in terza. Note that this strike should land on the same tempo with your pass.