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I have more familiarity with QML.
As far as I'm aware, you can only have one QQmlEngine running in a given process.
Now, Qt has a solution for this that is commercial/GPLv3 (not LGPL). That is the Qt Application Manager, which orchestrates multiple Qt QML processes.
https://doc.qt.io/QtApplicationManager/singlevsmultiprocess.html
I'm not actually sure if Qt for Python wraps that or not. @cmaureir do you know?