For the Swedish duo, use jj.

Jan Johnston with Tony Kirkhan were J.J., they released four singles and one album. The album is titled 'Intro' and came out in 1991. The four Singles are 'If I Never See Sunday', 'Slide Away', 'If This is Love' and 'Crying Over You', some featuring mesmerising acoustic and accapella performances on the B sides. Jan Johnston provided the haunting vocals with Tony Kirkhan supplying the melodic guitar.
http://www.janjohnston.com/

J.J. is also the name of an "Alternative R&B" artist from New Jersey. Read more on Last.fm. User-contributed text is available under the Creative Commons By-SA License; additional terms may apply.