"A" or "B" in switch statements?
As of Craft 2.6, this is now possible!
In fact, it works exactly like your first example:
{% case &quot;A&quot; or &quot;B&quot; or &quot;C&quot; %}
  {# run this if case matches A or B or C #}


Original answer...
Your first example is not currently possible, but might make for a good feature request.
Your second example is not possible, and most likely never will be. If it were added in the future, that would likely have a major (breaking) impact on all existing uses of the switch statement.
The way that Craft has implemented the case statement is by making it completely unnecessary to supply a break statement. In fact, the break statement doesn't even exist natively (though can be added with the MN Break and Continue plugin). The case statement will automatically end when it hits the next case statement, or the endswitch statement.
An actual solution would be to use a macro or include. If you have multiple case statements which should trigger the same behavior, I'd recommend wrapping that behavior into a macro/include, and calling that macro/include for each relevant case.