The cokriging equations are well-known, and you can reference them in Cressie (1993), for example. Any differences between SAS and Geostatistical Analyst are almost certainly due to different variogram parameters, different search neighborhoods, and/or different preprocessing techniques (transformations, detrending, declustering, for example).
What criteria are you using to say that SAS is more accurate? Are you using the default model and comparing crossvalidation results?