Unfortunately it’s not possible to affect a task’s availability based upon the status of a task in another project. Many users create “meta” tasks within the parent project that have a link to the other project.
It’s a shame, it seems cross-dependencies like this, between projects, would be fairly common. Unless I’m missing a philosophy in approaching this perhaps.
Thanks for the tip about linked meta tasks though, I’ll look into that and otherwise hope for a feature addition later.