Conditional linking between 2 parts - Simulink/ SimMechanics

Hi,

Is it possible to connect two parts in SimMechanics prescribing some conditions. Like I have two indivdual rotating parts which is connected again through a revolute joint. Now is it possible to ask the second part to start rotating after the first part rotates say 20°.

I'm using SimMechanics 2nd generation - R2013b. I want to create a conditional relation between the parts. Not sure how should i do this. or Is it possible in SimMechanics?