Content from two sections but one of which is related to a specific category I am looking to do the following.

I'd like to display a list of entries from two sections. That much is straightforward; however the kicker is that I only want entries related to a specific category of one of the sections.

So:


Everything from SectionA
Only CategoryC from SectionB


Possible?
You would do such things by first getting the entries IDs and then do a query to get all entries with these IDs:

{% set entryIdsA = craft.entries.section('SectionA').ids() %}
{% set entryIdsB = craft.entries.section('SectionB').relatedTo('CategoryC').ids() %}

{% set allEntryIds = entryIdsA|merge(entryIdsB) %}

{% set allEntries = craft.entries.id(allEntryIds).order('mySortByField desc') %}