When you move your slider, it calls the editor valueChanged() method, which in turns, calls the effect setParameterAutomated().
setParameterAutomated(), internally, calls setParameter(). This method is virtual, you should override it so that it sets the value of the parameter associated with the 2nd fader to whatever you want it to be. Just don't forget to call AEffectX::setParameter() too.
Then, whatever mechanism you have in place for notifying the UI of parameter changes will kick in, and your 2nd fader will move properly.
You have to do it this way, so that the 2nd fader moves, regardless of why the 1st fader moved (either because the user dragged it, or because the host sent automation events to it).