Hi,
you are talking about polyphase power converters?
Imagine:
You have tow instead of one converter, therefore the (ripple) current is divided by two.
And the ripple frequency is multiplied by two - the phase shift usually is 180°
All in all this gives about 1/4 of the ripple voltage at the same capacitor size.
I don´t think there is a relationship between interleaving and efficiency.
If you use the same devices, then the switching loss may be about doubled, but the conduction loss may be about the half.
But you may use more optimized devices for half of the current.
Klaus