You obviously need a trigger to match on your line, whatever that might be. And then you'd use SendToNotepad to send the contents to a notepad window.
If you want to have multiple logs at the same time, you have a few options. You can either save your notepad windows (SaveNotepad), open a log (OpenLog), write to it (WriteLog), and then close it again (CloseLog) per write. Or you can use FSO to write to each file.
With FSO, you can have multiple logs 'open' at the same time, simply with multiple FSO objects.
Some links:
http://www.gammon.com.au/forum/bbshowpost.php?bbsubject_id=1979
Points you to the FSO help file online, there is also an entry in the Windows Scripting help file (the VBScript/Jscript help file that Nick points to on the downloads page).
I cant seem to find the post in the forums that is an explination of FSO, but if you search for FSO (or FileSystemObject) youll return a few hits.
Obviously you can also search the forums for any of those script commands and get examples.