I was just curious so I tested it and it worked out nicely:

1) Finish China, so Garga is enabled
2) Buy SL 1 advancement, so Trisk is enabled
3) Apply 1 SL power (got it from the doors), so US is enabled
4) Finish Hard Scand to buy SL 2 advancement.
5) Voila, EH is enabled

So it does not matter in which order you apply SL powers and/or earn a new SL advancement level, they work together just fine adding to each other and not overwriting :)