As noted by @klanomath 'No file will be overwritten': You can have multiple JDK's installed but only one JRE. But each JDK also comes with a JRE. Apple's Java integration with OS X provides /usr/libexec/javahome that can help you keep the JDK situation straight.

Java 1.6 For Mac Ppc

That selects among the JDK's and their associated JRE found in /Library/Java/JavaVirtualMachines. See the man javahome page. However there is no equivalent capability to select among versions of the JRE accessed via /Library/Internet Plug-ins/ provides the only 'official' way to select among those, which requires terminal savvy with sudo and symbolic links. Since you are a developer testing web apps one assumes that will suffice, but not something mere mortals want to mess around with.