You definitely don't want >1 System object or >1 Renderer object, to have that requirement would be awful
What you're trying to do is (will be?) possible in 0.7.x. That 'x' is quite important because some stuff might not make the initial release in a 'friendly form' - I've made that decision because it became apparent that a release - any release - of the v0-7-to-be code was becoming essential, and as such certain supporting elements for some of the more advanced things in the new rendering system are getting bumped to later releases.
I'll look over this in the next couple of days and see what the obstacles currently are - one the one hand it should already be possible with some extra code, but on the other hand, I'm aware of various places where we do things which are not 'right' when considering multiple / secondary roots.
It's almost certain that when 0.7.0 is released, it will be possible somehow
, though a nice friendly one-call interface will probably follow in 0.7.1 or 0.7.2