Really depends on what you want.
You can use related files. To do this, simply create one “common” project with resources, and in the second project, which you use “Add an existing item”, before clicking “Add”, click on the small arrow to the right of the button and say “linked file”. This will not copy the file, but save it at the current location. But it will depend on him, as if it is a regular project file ...
If you just need static content shared across multiple websites,
a) have one website storing static content and link to all URLs to link to this static content website.
b) If you do not want to host a separate website for static content, simply create an additional website on each main website ...
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