Compositing two rendered images into one?

I’m attempting an animation with particle systems. I rendered half of the scene (the part without any animation), and then I rendered the particle system separately because I simply wanted to test its path etc. The image below on the left is the scene without the animated sky and leaf particles. On the right, the sky and particles. Now I want to view the flying leaves with the rest of the scene, and I thought I could simply put them together in the video sequence editor, but from what I’ve tried, I can only view one at a time. Is there any setting I’m missing in order for me to put them together?

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Use alpha over.