Not possible like that.
See https://secure.phabricator.com/T5474 for this very request.
See https://www.mediawiki.org/wiki/Phabricator/Help/Herald_Rules for what's possible in Herald.
See https://phabricator.wikimedia.org/T91716#2380691 for a potential related workaround.
FWIW, this can be worked around if the column is a Milestone, as Milestones are technically projects, and thus have tags that Herald can detect. The challenge is that this is using Milestones in a way they aren't really meant to be, so there are other restrictions that might become burdensome, such as the inability to move Milestone columns.
I don't like marking tasks as Upstream.
Well, I don't like sharing tasks with an independent projects in the first place. You may triage the Pywikibot-tests task as hight priority task, but the priority for the MediaWiki-API may be different.
Also it's usually better to have a more project-specific description for each project.
Therefore I think it's best to create separate tasks for each project and make one the parent of the other.
This will also reduce the notification noise for the developers of the other project as they won't see the updates of the other task (for example if Pywikibot-tests decides to skip some tests until the upstream task is resolved, there is usually no need for the subscribers of MediaWiki-API team to receive notifications about discussions and patches that lead to that).