Yes, the 9.0 and 8.x versions of the License Manager can be installed on the same machine.
A version 9.0 license file can be created using an existing hardware key number from a version 8.x or previous License Manager server. Once the version 9.0 license file is obtained, install the ArcGIS 9.0 License Manager on the same machine hosting the ArcGIS 8.x License Manager, making sure to use the same hardware key for both. Two hardware keys cannot be used on the same server.
<a href='http://support.esri.com/en/knowledgebase/techarticles/detail/26014' target='_blank'>How To:Install the 9.x ArcGIS License Manager and the 8.x ESRI License Manager on one Windows machine</a>
A version 9.0 license file can be created for an existing hostid and hostname if a user intends to administer both the version 9.0 and 8.x License Manager on the same machine. The version 9.0 and 8.x software must be installed to different directories and each sysgen directory should have only the proper version of the license file.
<a href='http://support.esri.com/en/knowledgebase/techarticles/detail/26017' target='_blank'>How To:Install the 9.0 ArcGIS License Manager and the 8.x ESRI License Manager on one UNIX machine</a>
- ArcMap 9 x
- Legacy Products