Wait until fragment has been added to the UI

Fragments lifecycle works in your favor here. According to the documentation the method onStart() is "Called when the Fragment is visible to the user", so I suggest you do something like this in your first fragment class:

public void onStart() {

    super.onStart();
    ((MainActivity)getActivity()).loadSecondFragment();
}

And in your Activity:

public void loadSecondFragment() {
    if (isLandscape) {
        openSecondFragment(mIndex, R.id.rightConent);
    }    
}

And voilá! Try any of the lifecycle methods and see which one works best for your purpose.