Title: E-mail from Richard Zimmerman, International Joint Issues Web-Site.
See the Joint Issue Web-Site 
To be accepted in the list of joint issues, the stamps must follow several rules among which the most important is the will of two countries or postal administrations to cooperate. Date of issue or design become then of second importance.

The Goethe issue is definitely a joint issue classified T3 (twin issue = same date + same design with in this case adaptation to local constraints, actually only the portrait is the same). Mixed covers (both stamps first day cancelled on the same cover from official origin), mixed cards and even mixed minister cards do exist. The story of this issue is particular as it was announced in Germany only a week or so before its issue. Actually its production was probably decided at the very last minute in conjunction with a political event involving South Korea either in Germany or in South Korea.

The other issues involving South Korea are to our knowledge the following:
1972 1 April with Thailand (probably "accidental" joint issue as several other countries (at least 5) were supposed to issue stamps at the same occasion on that day.
1988 6 May with Greece [T1] Olympic Games.
1995 18 December with Japan, 30 years of relationship [P1] same date different stamps.
1996 8 February with China, Optical Fiber, initially a twin issue, but technical problems prevented these similar stamps to be issued at the same date.
1999 8 December with Germany, Goethe.
2002 20 November with China, Taekwondo and Wushu (Kung Fu).
2002 21 December with Viet Nam, Dabo Pagoda-Korea and One Pillar (Mot Cot) Pagoda" in Hanoi, Vietnam.

The new classification system will be presented in details within the next days on our web page.

Best regards
Richard Zimmermann