When installing both, put the 64-bit JRE in C:\Program Files\Java and the 32-bit JRE in C:\Program Files (x86)\Java . Use a batch script to set JAVA_HOME specifically to the 32-bit path before launching your legacy app.
However, Windows often gets confused. If you install the 32-bit JRE after the 64-bit version, it may overwrite your file associations or add the wrong path to your PATH variable. 32 bit java runtime environment