As per release notes to make Multi JRE feature work with new Plug-in, users can add/remove installed JREs on the machine using The Java Applet Runtime Settings dialog box.
While on the Windows platform, all installed JREs show up in the list automatically but on unix one need to manually update the enteries using this dialog. But it's not working on unix and to make multi JRE feature work on unix one need to do the enteries manually inside the deployment.properties file OR one can do the enteries using JAWS runtime settings dialog.

Steps to reproduce:
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1) Install 6u10 and 5u10 on unix platform
2) Invoke JCP from 6u10 as follows
<jre_1.6.0_10>/bin/ControlPanel
3)Goto "Java" tab and Click "View" under Java Applet Runtime Settings
4)It should open up Runtime Settings Dialog for applets
5) Try to add the entry against installed 5u10 JRE
If it fails to add the JRE and deployment.properties file is not getting updated with 1.5.0_10 enteries then bug is reproduced