Open file1.txt using C-x C-f (find-file). Press M-! to execute shell-command. The command you want to execute is the copy:

copy /b file1.txt + file2.txt + file2.txt file1.txt

Reload the buffer using M-x revert-buffer. Beware! The copy command will overwrite file1.txt.

Open file1.txt using C-x C-f (find-file). Press M-! to execute shell-command. The command you want to execute is copy:

copy /b file1.txt + file2.txt + file2.txt file1.txt

Reload the buffer using M-x revert-buffer to see that indeed the files were appended. Beware! The copy command will overwrite file1.txt.

If you don't have file1.txt open, then the default-directory will not be for the correct folder. You would need to either issue cd first to change the default directory for the current buffer or use absolute paths.