essentially you can't - only one version of a particular assembly can exist in an application domain at any one time. If you load one, either declaratively or dynamically (either from bin or GAC, it doesn't matter), when you stop using it, it will still remain in the app domain and only an application domain recycle will remove it, so an attempt to load a 2nd (different) version of the assembly will fail.

In general, the best way to deal with 2 different versions of the same assembly is to use assembly binding redirects - where the request for an older version if "forwarded" to the newer version. You can see us using this in DNN with the telerik assemblies for instance. However this relies on the binding assembly redirect to exist, and that the newer assembly version is binary compatible with the older one, which is not always the case as some assemblies break compatibility.

As for assembly versions, yes, dnn does check for this -as long as the extension developer has correctly built their module i.e. to use an assembly component (http://www.dnnsoftware.com/wiki/page/...) as this allows for versioning and referencing counting and ensures the older versions cant overwrite etc.