Device: HTC Desire V (Dual Sim)
OS Version: Android 4.0.3
Avast Version: 2.0.3917
Avast is not able to block an incoming call from a number when the call is made to the Sim 2 Phone Number. It is able to block incoming calls made to the Sim 1 Phone Number. So, the issue (unable to block incoming call) exists only with the second sim number.
as far as I know, dual SIM devices are not officially supported in Android OS. Such devices exists and the support is somehow baked in by the manufacturers of those, but there seems to be no such support in common Android that we could use to handle multiple SIM cards.