How to add record in common collection using WPF MVVM from different view model?

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WPF

MVVM

I have one main window and containing some different user control. Structure like this :
TemplateWindow : this window contains Section user control. I added data template as there can be multiple dynamic section.

SectionControl: It contains dataTemplate for dynamic Question user control.Section contain data trigger for show proper question depending on type.

YesNoQuestion_UserControl
WhQuestion_UserControl


I am creating different view model for each means for TemplateWindow,Section and questions but I want one collection which is Template that contains multiple sections and sections contains multiple questions.

But I am not able to achieve this.