Okay, looks like I got bit by the "Reading Is Fundamental" bug...

The short answer is CALCULATETABLE.

When I actually OPENED Ferrari & Russo's DAX Patterns book, there's a chapter on Surveys (Chapter 13)... the question in the book is "Which customers gave the same answers for both Question1 and Question2?" and the answer is:

CALCULATE (

COUNTROWS ( Customers ),

CALCULATETABLE (

Answers,

USERELATIONSHIP (Answers[AnswerKey], Filter2[AnswerKey] )

),

CALCULATETABLE (

Answers,

USERELATIONSHIP (Anwers[AnswerKey], Filter1[AnswerKey] )

)

)

In case someone is looking for this, it's here[/url].

Yep, "Here's your sign."

... after more playing around with this... I got this working... I guess CALCULATETABLE is something I need to figure out...

-- has HF

CALCULATETABLE('Patient',FILTER('Diagnoses',[SymptomName]="Heart Failure"))

-- has D

CALCULATETABLE('Patient',FILTER('Diagnoses',[SymptomName]="Diabetes"))

-- has both:

INTERSECT('has D', 'has HF')

or

INTERSECT (

CALCULATETABLE('Patient',FILTER('Diagnoses',[SymptomName]="Diabetes")),

CALCULATETABLE('Patient',FILTER('Diagnoses',[SymptomName]="Heart Failure"))

)