Composite two scenes for only a short period? I have a scene with a particle emitter
and a second scene with the actual particles.

I separated them so I could have more control over motion blur.

When I try to render the animation, it renders both scenes on every single frame, but the particles are only in the  scene for a couple frames in the end.

So my question is: can I tell the compositor to render and combine two scenes for only a few frames at the end instead of rendering and combining both scenes for the entire clip? I imagine it would save render time.
You can enable and disable rendering Render Layers by clicking on the checkbox for a layer in the Render Layers tab in the Properties area.

You're also able to create keyframes for this setting by either clicking on the checkbox with the right mouse button or pressing I while the mouse cursor is over the box.