Recommended to use OPPS_MULTISITE_FALLBACK = True and have in mind that boxes will return a list of ContainerBoxContainers instead of a list of Containers.
To preload all the container boxes of a given page and avoid duplication of content the load_boxes tag should be used, this tag will take a list of slugs and the order of slugs in that list will be the precedence to avoid repetitions.
In the base template the following block should be defined.
# templates/base.html