I've always taken twinbossing to mean a mixture of embossing and debossing such as on the examples listed in my previous post . . . raised designs with recessed/sunken parts:-
1) INFINITY WAR - Thanos embossed / jewels debossed
2) DEADPOOL - "D" (on both sides) and title embossed / eye debossed on both sides
3) GANG RELATED - badge embossed on back / "US" in centre of badge debossed
4) GREAT MOUSE DETECTIVE - characters embossed / title debossed
Regarding different levels of embossing - that's simply called multi-level embossing and is not related to debossing and is as rare as twinbossing on steelbooks and more likely to be seen on futurepaks.
I suppose to be strictly accurate though I have to take into account how the manufacturer describes the term in their listing of for example MOJIN: THE LOST LEGEND
"The Lost Legend SteelBook® edition. This beautiful edition is twinbossed to get a dramatic effect in artwork: The skeleton eyes are debossed giving them extra depth, while the golden title treatment is embossed."
Here the embossed title speaks for itself but the debosseed 'eyes' are not part of the embossed artwork such as in examples 1 - 4 above:-