Use a script trigger, I think OnObjectEnter on the portal object will work, with Get ( CurrentPortalRowNumber ) to save the row number in a global field. Your script can then use go to object to put the focus on the other tab's portal and then go to portal row with a reference to the global variable to scroll the portal row into view.
Unfortunately, while this makes the portal row visible, it can't always position the selected portal row in exactly the same position as was shown in the other portal.
There is a way to use calculation fields and global variables or fields to dynamically sort a single portal instead of using two portals. This method isn't always feasible--the portal can be slow to update due to sorting on an unstored field, but does avoid the issues you are having here.