I use the "start=" option of the Image command in order to display a certain document when the user clicks on the image. Is it also possible to display more than one document?

Yes, please specify a .bat file in the start= parameter, and then display the documents in this .bat file.

You can also pass parameters to the .bat file, e.g.

Image ... start="showdoc.bat  &F[Material]"

Refer to these parameters with "%1", "%2",... in the .bat file.