You'll need to activate the meta flag for controls that are to be modified by other controls like this:
Code:
GetParam(kParameterNameL)->SetIsMeta(true);
GetParam(kParameterNameR)->SetIsMeta(true);
And then in OnParamChange set them like this:
Code:
case kParameterNameL:
GetGUI()->SetParameterFromPlug(kParameterNameR, GetParam(kParameterNameL)->Value(), false);