Measure with different outputs based on condition

Anonymous Try:

Measure =
  IF(
    MAX('Table1'[Column]="KPI1",
    DIVIDE(Sum(Num), Sum(Den), 0),
    DIVIDE(Sum(Num), Average(Den), 0)
  )

Try this Anonymous 

MeasureName =
VAR varValue =
    MAX( TableName[Field] )
RETURN
    IF(
        varValue = "KPI1",
        DIVIDE(
            SUM( Table[Num] ),
            SUM( Table[Den] ),
            0
        ),
        DIVIDE(
            SUM( Table[Num] ),
            AVERAGE( Table[Den] ),
            0
        )
    )

You need to convert the field to a scalar value, which MAX does when used like this. MIN will give you the same result.