I think to implement you need http2, and use server push to send possible multiple responses.
Looks like it requires clients to have a specific data model to, because the additional results are sent back as "patches" to the initial result
The only info i have is a couple of talks from React Conf, that describe the high-level concept.
Well an easy data model is to mark the initial response with “placeholder” elements, then the patch can send a path and data for each of those. After all, the graphql response does have a canonical and well defined format.