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From the InkToolbar.TargetInkCanvas Property you could see that this property is an InkCanvas object. That means the InkToolbar could be bind to only one InkCanvas at the same time. So you could not set a global InkToolbar for multiple InkCanvas at the same time.
Currently, what we could do is change the InkToolbar.TargetInkCanvas Property to different InkCanvas when needed so that the InkToolbar setting could be applied to different InkCanvas.
Please check the document here: https://learn.microsoft.com/en-us/uwp/api/windows.ui.xaml.controls.inktoolbar.targetinkcanvas#Windows_UI_Xaml_Controls_InkToolbar_TargetInkCanvas
https://learn.microsoft.com/en-us/uwp/api/windows.ui.xaml.controls.inktoolbar