Unfortunately wiki documents and discussions can only be created in a single specific space. The primary reason is that each space has its own set of permissions, allowed content types, etc. With that goal in mind, the logic around creating and viewing content living in multiple spaces could be counterintuitive.
But this problem is the ideal use case for tags. Think of a space as a folder on your operating system. It can have sub-spaces (nested folders) and some files (wiki docs and discussions). Each file must live in one and only one parent folder. We are all very comfortable with this paradigm, but the drawbacks become self-evident in time.
Let's take a simple example. I want to organize all of my recipes, so I create a folder (space) called recipes. Now what? Do I create more sub folders based of the type of food, such as Italian, Chinese, Indian, Mexican, etc. Or do I create sub folders for breakfast, lunch, dinner, and dessert? How do I classify an Italian dessert then?
The solution to this problem, at least in Clearspace, is to use tags and tag groups. So wiki documents and discussions must live in one and only one space; however, they can be tagged with any keywords you might use to look for them. You can then browse the content by tags, regardless of which space the content was created in. Tags will transcend this typical folder-tree structure that we are all used to, and provide a means to find relevant content regardless of where it is created.
Also, we went a step further and created tag groups. A tag group is nothing more than a named collection of tags. The primary benefit of a tag group is that it groups your content virtually. A wiki document or discussion is not created in a tag group,
but will be associated with one or more tag groups if it is associated with tags in the tag groups. If a discussions morphs into something entirely different in time, you can change its tags to change the tag groups it can be found in.
So when would you use a sub-space as opposed to a tag group? A good rule of thumb is to think about people as opposed to content. If you simply need need a way to organize content, use a tag group. If instead you need to capture permissions for specific users, limit available content types, and so on, then you should create a sub-space.
Here is an example of both:
Space: Product Development
Sub-spaces: Clearspace, Forums, Openfire, Spark
The preceding sub-spaces are some of our different products, and they would be sub-spaces because there are different teams with different permissions for each.
Tag groups: Release Plan, Weekly Status, Documentation, Features
The preceding are tag groups because they are simple containers of content; they do not need all the extra complexity a sub-space provides. As mentioned above, it is entirely possible to create a document or discussion that can be found in both Documentation and Features simply by tagging your content with the appropriate tags, say "documentation" and "features."