Currently Notepad2 allows dropping only 1 file from Explorer and shows an error otherwise. Instead of the error, let it work this way:

• first, like with dropping 1 file, close current file (asking to save it, etc. as if user called File > New)
• take every dropped file one after another
• load every file as if it was opened as usual in Notepad2 (FileLoad or FileIO); this can bring interactive prompts (e.g. for large files)
• after loading (with the usual charset and EOL detection, etc.) append the entire buffer to a common large buffer
• after processing all files create a new file (as if via File > New, i.e. with default charset, highlighting and other settings) and paste the large buffer into it
• any error during processing (loading of any file, etc.) stops it and displays an error (either leaving old file opened or creating a new blank buffer as if after File > New)

Essentially this works like a *nix cat command or Windows copy file1 + file2 + ... except that it can load files with different charsets, etc. as long as Notepad2 is able to correctly detect each file's options.