J.J. & Jeff is a platform game released in the US for the PC-FX. Information on this title is currently limited.  Platformers were a common genre at the time, and J.J. & Jeff likely presented standard platforming mechanics, possibly with unique character abilities or level design.   More information on its developer and specific gameplay elements would be necessary for a more complete description. 

J.J. & Jeff (USA)

Image Fight (Japan)