The reason it doesn't work for you is that the data stored in the field when both check boxes are clicked is either:
Which value is listed first is determined by which checkbox is clicked first.
You can see this for yourself if you put a copy of this field next to the one formatted with checkboxes, but formatted as an edit box, then click the boxes and what what values appear in the edit box each time you click a check box.
Here's a test you can use for the case when both values have been checked:
Not Isempty ( Filtervalues ( Restitution ; List ( "Initial" ; "Final" ) ) )
This will be true if both Initial and Final have been selected and the order will not matter.