The Appellant was arguing that being tried with other accused not charged with the same crimes could prejudice the assessment of his criminal responsibility for the crimes for which he was charged. The Appeals Chamber affirmed the Trial Chamber’s finding:

21. The Appeals Chamber recalls that the Trial Chamber noted that a common feature of joint trials is that evidence brought relating to one accused may not relate to another. However, the Trial Chamber found that this fact in itself, “unsupported by concrete allegations of specific prejudice that is likely to result,” does not mean that prejudice to an accused is an inevitable result of joinders under Rule 48.[1] This is because the Chambers of the International Tribunal consist of “professional judges [who are] able to exclude that prejudicial evidence from their minds when it comes to determining the guilt of a particular accused” in a joint trial.[2] The Appeals Chamber affirms that holding.[3] 

See also para. 22.