Per T182548, getting all the data for InteractionTimeline from the MediaWiki API isn't going to scale for users with lots of edits. We need to build our own API to query the replica databases directly and thus speed up getting results.
The API should be able to return all the revisions performed by two users on overlapping articles for a specific wiki within a specified time range. In other words, it should accept the following parameters: user1, user2, wiki, start, end (and perhaps limit). It should return results in whatever format will be easiest for the existing InteractionTimeline codebase to process.
The API should live on ToolForge in the interaction-timeline project.
See https://wikitech.wikimedia.org/wiki/Help:Toolforge/Database for relevant documentation.
[Feel free to rewrite this description as needed prior to estimation.]