My understading in this issue is that you get whatever you want as long as you can prove the feds that you can do it safely and that you follow and meet all their requirements. I was involved in a 121 supplemental, but, we also had authorization for some schedule flying. Having a good relationship with your POI (Primary Operations Inspector) makes the BIG difference.
You can definetly have 2 different certificates, but remember that the airplanes can be only for specific certificate. They cannot be in different OPS SPECS...
Hope that helps...
