Edit the list entry that you want to appear current for the multiple pages. On the list entry page there is a "Current List Entry" section.
Set the "List Entry Current for Pages Type" field to "Colon Delimited Page List".
Set the "List Entry Current for Condition" field to a colon delimited list of the pages you want the entry to show current for. So if you want it to be for pages 3 and 4 enter "3:4".