Sum Multi tier selection

I have two multi selection lists, one dependent on the first. A few questions,

How do I filter the second list based on the multiple selections from the first list
How do I then loop through the multiple selections from the second list against the same column on another sheet and sum values

Example:
List One is Org Level 1, and should allow for multiple selection
05
06
10
18
etc..

List Two is Org Level 2 and there is a one to many relationship to Org Level 1 - again, multiple selections allowed here
Org 1 of 05 has Org Level 2 of 0510, 0520, 0540 etc..

I am trying to sum active employee count based on the selected Org Levels - this data looks like the following:

Org Level 1 Column A Org Level 2 Column B Headcount

05 0510 23
05 0520 5
05 0540 4
06 0603 18

And so on...

So I want to loop through the multiple selections made in the second list, and sum headcount

Many Many Thanks!!!!

Thanks - here is an example...