Branching stacked pull requests (multiple chains with shared base)

[jonnyandrew]

Would it be possible for stacked pull requests to support branching / non-linear stack structures?

That is, multiple pull request chains that share the same base:

                  - PR3 - PR4   (feature A chain)
                 /
main - PR1 - PR2
                 \
                  - PR5 - PR6   (feature B chain)