They explained the problem, which was latency: 300 milliseconds or something if they would play on the same 'live stream' but were in different homes, and they solved it by playing through multiple streams while using a metronome?
As I have no knowledge about music this is the best I can re-tell it :D It did work though, they actually played 'together' and the music between them had no lag :-)
for my knowledge this sounds like a great achievement :D