How to install Java 7 next to Java 6?
The oracle Java 7 does not remove the Apple supplied Java 6. It does however change /usr/bin/java to start the Oracle Java.

To start a jar with Java 6 instead of just using java on the command line put in the full path to the java i.e. /System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home/bin/java