As to the ID3 (informal) standard: It allows for multiple strings for any text frame. It is only not allowed to have more than one text frame with the same id in a track.
Quote:4.2. Text information frames
The text information frames are often the most important frames,
containing information like artist, album and more. There may only be
one text information frame of its kind in an tag. All text
information frames supports multiple strings, stored as a null
separated list, where null is reperesented by the termination code
for the charater encoding.
