Javascript files and images can be placed in a common place - for standard javascript files (jquery library, etc.) it is recommended to use CDN, as Kiron points out, you get many advantages there.
For other content files, you can simply place them on a common URL that both sites access - for example. with two sites at different URLs:
http://site1.somedomain.com/default.aspx
http://site2.somedomain.com/default.aspx
they can use content from a common location, for example:
http:
http:
The same thing works with virtual directories, not fqdn.
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