Yes, Two different versions can be installed under a single AIX server. Infact not only two, one could also install three different versions 8, 9.1, 9.5 as three of them take different paths by default. Version 8 gets installed under /usr/opt/db2_08, version 9 under /opt/IBM/db2/V9.1 and version 9.5 under /opt/IBM/db2/V9.5. One installation shouldn't disturb the other installation. Because all the three versions gets installed in a different directory and are independent to one another, it is possible to create three different instances on one AIX server using three different versions. For example, db2inst1 -- using V8, db2inst2 using V9.1 and db2inst3 using V9.5