if u select side view and drag the the box over the vertices they should be selected on the other side also...but sometimes they may not be lined up so this doesnt always work so u would go into persepctive view just to check. Just clicking on a vertex point wont select the others opposite only dragging the box will thenonce u have some selected to ADD to ure selcetion u will hold Shift. been careful not to resect already selected vertices as hold shift deselects them.