You can also do this using mmark
Save the main content file with .mmark extension and then inside the .mmark file you can include text from anywhere - the contents of the other file will also be treated as markdown - mmark will include text inside {{}} e.g.
{{/path/to/some/more/text}}
For this to work you need to rebuild public with Hugo when the text changes - watch will not dynamically update if the included text changes - it is only rendered when you build with Hugo