Fork a running build

Running 0.7.3, but appears to also be the case with 0.8.1 as well. I’d like to start a new (fork) build with specific non-default parameters, triggered from another repo using the downstream plugin. It doesn’t seem I’m able to start a build if the parent build is still running. Why does that limitation exist? Thanks!

drone build start -p fork=true foo/bar 189
client error 409: Cannot re-start a started build