Take a look here..
http://java.sun.com/j2se/1.5.0/docs/guide/plugin/developer_guide/using_tags.html#html
If the following classid value is used in the object tag,classid="clsid:8AD9C840-044E-11D1-B3E9-00805F499D93"
then the end user will use the latest installed version of the plugin.
For a specific version, say 1.5 useclassid="clsid:CAFEEFAC-0015-0000-0000-ABCDEFFEDCBA"
So your html code would contain two object tags, each with a different classid value
as well as code value.