TYPE B.TXT>>A.TXT should append b.txt to a.txt (caps are for emphasis only)
Copy a.txt+b.txt requires a new output file, a file cannot be copied onto itself.
e.g. copy a.txt+b.txt c.txt (c.txt would contain the appended files.)
This is not a recognized use for the Append command, enter Append/? at the Command Prompt to check the syntax.
Good luck