You could use the ApplicationPackage Tag property in order to share data across XAP. In the external XAP, get the ApplicationPackage which has the Tag information modified using the UXShell object. Here is the snippet:

string tag2 = UXShell.Current.GetApplication("ClientUIExternalApp2").Tag.ToString();
string tag = UXShell.Current.GetApplication("ClientUIExternalApp1").Tag.ToString();
uXTextBox1.Text = tag + " || " + tag2;

Good question. In addition to using Tag as Glenn's reply, you may want to save the application-level settings in isolated storage.

The ClientUI Application Framework enables you to do that pretty easily. Check out How-to: Save and Load Application-specific Settings.

Hope this helps,
Jimmy