I think you are doing something wrong: I just tried it by copying a 1.01GB file into a "spare" folder twice and renaming them to file1 and file2.
Then in a command prompt I changed to the appropriate folder, and entered:
copy /b file1+file2 outputfile
It took a little time, but it worked fine and I ended up with two 1.01GB files, and 1 2.02GB file.
So what am I doing that is different from you?