nice memory BFL!
the Guiness record was for the McGuire (or McCrary) twins. they were the "world's heaviest twins" and famous for the photo on motorcycles...
http://1.bp.blogspot.com/--co59Wi3Qu...e073372c10.jpg
it certainly appears the photo below was used for the shirt back in '92, and one of the images was duplicated to reflect three of them for the beastie boys...
http://4.bp.blogspot.com/-02wBARM6s9...uire-Twins.jpg
http://en.wikipedia.org/wiki/McGuire_twins