I think you need to take a step back here and understand whats going on Sansa before you decide to use CSPSHARE=1. CSPSHARE should only be used as a last resort.
Firstly, we need to clarify what you mean by 'Namespace' when you are using CSP/ZEN pages in Cache. I think what you mean by 'Namespace' is actually web Application. For example, accessing the page:
Uses the /csp/samples web application (which speaks to the SAMPLES namespace), and using a url like:
Is using the /csp/user web application (which speaks to the USER namespace).
When you add the CSPSHARE=1 parameter, it bypasses this (as well as several other security checks like whether or not the same browser made the request as previous requests for this session). Now simply having the same CSPCHD parameter in the request is enough to make Cache think it is the same session (pages with CSPSHARE=1 automatically add this parameter to all of their links). So yes, now you can share data between the pages using %session, but you have also opened up a whole bunch of security related sharing between the pages that you may not want.
There are many ways to share data between pages without having to use the CSPSHARE feature. If you want to make use of the %session variable, then take a look at this section of the documentation that describes methods of sharing the authentication context and its associated data:
There are other methods as well though which don't touch the authentication/session stuff and let you just pass data between pages. For example, you could just pass the data in the url parameters, and if it's sensitive data, you can make use of the ENCODED parameter. You could also store the data in a global in Cache and access it from both pages.
Hopefully this gives you some options. If nothing else gives you the behavior you want, then go ahead and use the CSPSHARE feature, but be aware of the drawbacks of using this feature, and I would recommend trying everything you can to avoid using it if possible.