How can I use an existing page selector in a dashboard, to filter a line item that is using the same list? I have created a module that uses a country list as a dimention. Then, I created a dashboad to pull this module and used the same list as page selector. Now I have created an other module, but on this module I am using the same country list data line Item. I am planning on addidng all of them on the same dashboard, how can I do that?
You can't reference a page selector in a filter, unfortunately.
You might be able to get the functionality you want by adding a Boolean line item dimensioned by the same list (as well as everything else in the module's dimensionality), then putting a formula in that line item that checks if the row you're looking at has that item in the filter. You could then put your filter on that line item instead of the list-formatted line item, and it will pick up its context from the page selector and change the visible rows accordingly.
Be careful this approach doesn't create a size problem for you though.