https://www.youtube.com/watch?v=Nol8kKoB-co
I believe Joanna from Qubes also set-up this forum for discussions on secure operating systems:
Joanna also talked a bit about the trade-offs between the two here:
https://secure-os.org/pipermail/desktops/2015-October/000002...
I believe initially there were some discussions to integrate Subgraph into Qubes as a TemplateVM (just like the Debian VM, Ubuntu VM, etc), but the Subgrapth guys thought Grsecurity wouldn't work well with Qubes OS. I think that situation has improved, and there is some progress in making Grsecurity work with TemplateVMs and AppVMs.
https://twitter.com/Phoul/status/801114260881424384
However, even if it does work, I'm not sure how excited the Subgraph guys are about making their OS "just" a Qubes OS TemplateVM. They may think that's the wrong business strategy for them as a company. I'm just saying this as someone watching from the outside. They may actually not believe that at all.
However, I did also notice the relationship between the two projects got a little colder, at least for a while, and in public, after Edward Snowden called Qubes his preferred secure OS.