Recently we had a couple of clients inquire why xAPI course registrations when launched concurrently through Engine, were not getting the concurrent launch warnings.
We only support concurrent launch detection in Engine for SCORM courses. Mainly because with xAPI launches, we don't have a player that informs us when the the user has exited (so every launch would look like there were other launches still open).
Also, the API for receiving statements is not setup to try and match the statements it receives against a particular launch and whether that launch is still 'valid' (eg, not the 'old' one), in the way that Engine does for data sent from SCORM courses that have been automatically ended due to a concurrent launch.
xAPI courses really are a different kind of animal than SCORM ones, even though we smooth over some of the differences with the launch process.
If you have any questions about Engine or any of the learning standards it supports, feel free to shoot us a message at firstname.lastname@example.org.