When Installing java on 64 bit windows platforms the registry is written such as to make the javaws.exe being installed the default handler for jnlp mime-types.  
Unfortunately this applies to both 32 and 64 bit browsers, so when installing the 64 bit Java the 64 bit javaws becomes the default handler even if the 32 bit version were already installed.
Since many applications have native libraries in only 32 bit, it is preferrable to use the 32 bit version of java web start when both are available.  
This can be done simply by not configuring javaws in the 64 bit installer if the 32 bit version is already configured.
Care would have to be taken how to regonfigure when the 32 bit java is uninstalled on systems that have the 64 bit still installed.

An even beter way to handle this is to build in interoperability between the 64 and 32 bit javaws launchers, so that either could use either 32 or 64 bit jres depending on a preference indicated in the jnlp file