The short answer is that you cannot. Since QlikView Server are a set of Windows services, you should tweak a lot your registry (if possible at all) so you can have two instances of the same service running at the same time.
That's right. I read QVS as QlikView Server. But QlikView Desktop can coexist provided you set a different folder for each version you want to install. Note that the last version you install will be the default version that will open QlikView documents, otherwise you will have to modify the Registry.