Selecting a subset of entries, based on a relatedTo call
Second relatedTo call overrides first one. Calling it again won't add more parameters to the query.

If you don't want to repeat yourself you can always do this:

{% set creatorRelated = { targetElement: creatorEntry, field: 'creator' } %}

{% set caseEntries = craft.entries.section('case').relatedTo(creatorRelated) %}

{% set caseEntries = caseEntries.relatedTo('and', creatorRelated, { targetElement: categoryEntry, field: 'caseCategories' }) %}