All non-first-strike combat damage is dealt simultaneously, and all first strike combat damage is dealt simultaneously as well, in the step before normal combat damage is assigned and dealt.
This means that
Karlov of the Ghost Council will only trigger once in the first scenario, and twice in the second, regardless of how
Cairn Wanderer's damage is split among the blocking creatures and the defending player.
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#mtgrules chat room,
Cranial Insertion, the
Ask A Magic Judge Tumblr,
r/mtgrules, or even the Rules Q&A areas on the larger Magic forum sites will provide answers that are just as correct and informative, and often far faster than you'll receive them here. There's less than a handful of people able to post answers here, while more open venues often have multiple people hanging around specifically to answer whatever questions might happen to be asked at any and all hours of the day.