tl;dr:
If, for whatever reason, progress on the torvalds/linux.git repository is affected, someone has to get things moving within 72 hours. This person could either be the one who organized the most recent Maintainer Summit or the chair of the Linux Foundation's Technical Advisory Board (TAB).
They will be tasked with pulling together the people who were invited to the last summit, along with members of the TAB.
It gets more complicated after that, but essentially that's it.
Oh, and I'm pretty sure there already is more than one top maintainers for the kernel code anyhow.