Yes, but the templates will be overridden, and the default references in VS and Blend will point to the last installed API (to select v1.1 assemblies you have to explicitly browse to them and select them when adding a reference).
You can, they will both show up in the Add/Remove, but you will only see 1 template. Also, this will cause problem when trying to remove, so I would not recommend it, since I am currently having issues with removal.
