Following the discussion in T211800, assuming for now a single entity type can only be provided by single Wikibase instance, make it possible for wikibase to access entities provided by multiple sources.
For now, only direct database database access is considered.
The general idea would be to define "sources" that are providing entities of particular type. Services providing access to entity data will be defined per source (i,e. a particular service is configured to access the database of given source/instance/repository). Wikibase will determine which source-specific service to use to access data of particular entity based on the entity-type vs source configuration.
Entity ID prefixes are not considered for now, as long as there is no technical need (state January 2019) to have a single entity type provided by multiple Wikibase instances/sources./databases.