It is not possible by design using /writeini since /writeini take the first parameter (parameters are tokenized by space in commands) as the section's name.
It would be possible to use /write to write to the ini file and to construct the ini structure yourself, but that's no so great.
Another, easier solution, consist of replacing the space in the song with a character that won't be or is unlikely to be used in the title + artist, but this breaks if that character happens to be there, a character that could be used is the non breakable space $chr(160).
A 100% working solution would be to change the data storage but there is unfortunately no good data storage available in mIRC which would allow you to use this kind of relationship, that is, associating something which can contain spaces to some others data.

#mircscripting @ == the best mIRC help channel