Combining different extension Types
Right now SharePoint Framework Extensions support three different types of extensions: ApplicationCustomizers, FieldCustomizers and CommandSets . Can we combine two extensions under one project. Example: Based on selected List items in the list, do some customization in the header bar using Application Customizer. Please suggest.
This is absolutely supported and you can have as many extensions or web parts in single solution talking between each other. There are numerous options for the components to communicate with each other. One of such options is demonstrated in following sample in SharePoint GitHub – https://github.com/SharePoint/sp-dev-fx-webparts/tree/master/samples/react-rxjs-event-emitter. Since this is not really a new feature request, we will close the issue. Would suggest having questions and discussions in StackExchange, GitHub or in SP Tech Community.