With Microsoft Dynamics AX 2009 we do not support the setup where we have several instances of SQL Reporting Services on the same box connecting to different AOS. The reporting extensions always made connection to AOS mentioned in current configuration taken from Dynamics AX 2009 Configuration Utility. There is no way to change it. The only solution is to have several machines (virtualization).
Fortunately with Microsoft Dynamics AX 2012 it is changing. We have the way to implement it. The only requirements is that we have separate instance for each AOS to which we want to connect. More info you can find on TechNet site: http://technet.microsoft.com/en-us/library/hh389760.aspx