Based on the available code in here, i was able to merge the content of 2 docx files.
Unfortunately, the code doesn't work properly for docm: the merged generated docm file has unreadable contents.
Word found unreadable content in.....
After choosing 'yes' option, the content is shown correctly.
I saved the current version and did the comparison between the origin and the fixed one by inspecting them internally with ZIP.
What i found: docm doesn't seem to work with altChunk, because the fixed version removes it from the document.xml.
But Im not really sure, where does the problem come from ?