One way I've solved this is to open the second PR against the branch for the first PR, and explain what's going on in the PR message. Then, once the first PR is

One way I've solved this is to open the second PR against the branch for the first PR, and explain what's going on in the PR message. Then, once the first PR is merged, update the second PR to be against the real target branch. This works better with a system that requires the PR owner to actually hit merge (so it's not accidentally merged into the first PR's branch) and where approvals stick across PR changes (which is undesirable in general, though).