In Blender's manual (v2.78) it says that it is possible to have multiple rigid body worlds: https://www.blender.org/manual/physics/rigid_body/world.html.
I figured that this would enable me to have multiple independent rigid body simulations going on, but that doesn't seem to be the case, as one may only set one active group in Scene>RigidBodyWorld.
Or alternatively, I was hoping that this would enable me to bake one rigid body world group which is ought to simulate at one point in my total animation, and use a different world for a different rigid body simulation which should play at some future time in my total animation. However, this didn't work out either, as there is only one cache: if I change the active rigid body world group, the current bake gets transferred to the objects in the new rigid body world group.
It'd be nice if these independent simulations could have a different "Speed" value as well.
Is it somehow possible to have multiple independent rigid body simulations?
Or is it somehow possible to bake one rigid body world (apart from bake to key-frame, which finalises the simulation) in one cache (just like there are multiple caches for cloth simulations), and another rigid body world to another cache, and then have them both play if the animation is played (Alt+A)?
My current work-around (not ideal):
Have all simulations live in the same rigid body world, and use key-framing on the "dynamic" box to activate/deactivate the independent simulations. This is not ideal, as I have to re-simulate everything if the bake is freed.