You need Extend() function, or use  at non repeating field as
And, I doubt you use X relation to make the value list. Use ValueListItems() function instead of List().
As it turned out, the expression I posted above actually was working. The problem was that that expression was the consequent of a conditional (if-then), and the antecedent of that conditional needed the Extend function wrapping each field it referenced. Thanks for mentioning the Extend function. This now works:
Extend ( Field_1 = x ) and Extend ( Field_2 = y ) ;
GetValue ( List ( SourceField_1 ) ; Get ( CalculationRepetitionNumber ) ) ;
GetValue ( List ( SourceField_2 ) ; Get ( CalculationRepetitionNumber ) )