Saw this pass by on my feed. Essentially when connecting or using different airlines/operating ST carriers, you should now always only need 1 app.
Doesn't (yet) seem to work for full tickets on airline Y with app X, but still quite nice.

I'll trust it when I'll see it...

I had tried it on a mixed VS/DL trip using the VS app however a pop up message said for seat changes or to add/remove luggage I'd have to go to the DL app/website so it literally only gives a boarding pass

You just confirmed what I thought...

The video was posted 2 weeks ago, but 3 weeks ago I still had to do the check-in on UX and SV on their respective apps even though it was ticketed and marketed by SV.

Also got an email from UX about the check-in ("now possible to check in for your flight to MAD") while it very much wasn't possible on the UX website until SV checkin also opened up.

I'll see what happens next week for the return flight, same itinerary but in reverse

OLCI opened for SV SV UX, did OLCI on SV website, received a 'partially checked in' message (SV checked in, UX not yet). Tried to OLCI on UX, kept on getting an error (try again later). Decided not to care and to just get the bag drop agent to handle it for me as at least I was checked in for the first two SV flights and had checked luggage anyway + already had seat selected on UX (via the Tarom website, as neither UX nor SV allowed me to select seat beforehand on the UX flight, but for whatever reason Tarom did offer me seat selection even though they had nothing to do with the entire booking).

Arrived back home, luggage did not, got stuck in JED. Was transported to MAD on the flight the next day, then another full 2 days to be transported from MAD to VLC in the early morning and delivered at home in the evening. Miles and XP for first SV flight posted same day, second SV flight and UX flight haven't credited yet, guess they're following the same delayed route my luggage took /s