Sorry for resurrecting this old thread, but while reviewing my post history, I came up with an alternative solution that might be better depending on your needs.
As you mentioned, you were interested in compositing images from another scene. My original response made the assumption that you had already rendered your images from the other scene. If that wasn’t the case, you could use Blender’s Scene data block and the Append or Link operation to make your compositing workflow more seamless. This will allow you to press render once and have all data available to you rather than having to save out external files for each additional scene you want to composite from.
Blender organizes information about your project into data blocks (https://docs.blender.org/manual/en/latest/files/data_blocks.html). Data blocks can be shared between projects either by Appending (copying all the information from one data block and all subordinate data blocks) or by Linking (referencing the information in another file without making a copy). There are benefits and drawbacks to either, and you’d need to choose which option is best for you (https://docs.blender.org/manual/en/latest/files/linked_libraries/introduction.html?highlight=append).
So, what you would do would be to choose Append… or Link… from the File menu, double-click the file you would like to composite images from, double-click the Scene data block folder and choose the scene data block you would like to append or link (unless you changed it, its default name is “Scene”). Now you have added a new scene to your project’s .blend file and can use it in the compositor. At that point, you would just add a new Render Layers input node type from the Add menu in the compositor. From the Scene selection dropdown menu, select the newly appended or linked scene, and you now have that scene’s data available to you for compositing like the following image:
There are some things to consider, though. When you render from your compositor where both scenes are referenced, the frames that render are in sync. That means that if you render frame 86 in one scene, the linked or appended scene will also render frame 86. For compositing static scenes, this isn’t that big of a deal, but if you have animations or simulations, you may run into issues. There may be either a work around or workflow setting that accounts for this synchronization, but if so, I’ve never come across it. Hope this helps!