Make sure both pivot table share the same pivot cash. Then you can put a Slicer on one and connect it to the other as well. Then, one slicer will control both pivot tables.
In the picture below I created two pivot tables based on the same data table. Then I put a slicer on the first. Select the slicer and then 'Report Connections' on the Slicer ribbon. Check both pivot table names and the slicer will control both. Even when they are on different sheets.