Cost can be an issue. As for the latter issue, we've had this issue with DC18 (higher) and DC15 (lower). However, their interests and talents are in completely separate areas, so we've tended to focus on their passions and achievements in separate disciplines (humanities for DC18 and math for DC15).
As a PG youngest myself, it would have saved my family a lot of trouble to have me tested earlier, rather than after horrible experiences at school and me starting to get into a lot of trouble. Our oldest was HG (two others GT), and it became a point of tension, as two of our middle siblings were not. I think more of it was an issue when report cards came home or someone skipped a grade; most of the time, none of us cared, though.
