Hey, former software engineer at the Guardian here. Yes the news outlets are collaborating on stories too big for a single one.
The last I can remember was the Panama papers, which followed a very similar process. I seem to remember they all synchronized through the ICIJ [1], and more or less each journalist would cover their own territory / domain. Then they agreed on a reasonable date to release the news.
They shared more than just information, but also technical infrastructure to do the investigation.