diff --git a/ApplicationCode/FileInterface/RifDerivedEnsembleReader.cpp b/ApplicationCode/FileInterface/RifDerivedEnsembleReader.cpp
index c3c63b0bb9..3de8152683 100644
--- a/ApplicationCode/FileInterface/RifDerivedEnsembleReader.cpp
+++ b/ApplicationCode/FileInterface/RifDerivedEnsembleReader.cpp
@@ -24,7 +24,8 @@
 ///
 //--------------------------------------------------------------------------------------------------
 RifDerivedEnsembleReader::RifDerivedEnsembleReader( RimDerivedSummaryCase*     derivedCase,
-                                                    RifSummaryReaderInterface* sourceSummaryReader1 )
+                                                    RifSummaryReaderInterface* sourceSummaryReader1,
+                                                    RifSummaryReaderInterface* sourceSummaryReader2 )
 {
     CVF_ASSERT( derivedCase );
 
@@ -32,10 +33,20 @@ RifDerivedEnsembleReader::RifDerivedEnsembleReader( RimDerivedSummaryCase*     d
 
     if ( sourceSummaryReader1 )
     {
-        // TODO: This is assuming that the addresses of both reader interfaces are equal
         m_allResultAddresses = sourceSummaryReader1->allResultAddresses();
         m_allErrorAddresses  = sourceSummaryReader1->allErrorAddresses();
     }
+    if ( sourceSummaryReader2 )
+    {
+        for ( auto a : sourceSummaryReader2->allResultAddresses() )
+        {
+            m_allResultAddresses.insert( a );
+        }
+        for ( auto a : sourceSummaryReader2->allErrorAddresses() )
+        {
+            m_allErrorAddresses.insert( a );
+        }
+    }
 }
 
 //--------------------------------------------------------------------------------------------------
@@ -91,18 +102,3 @@ RiaEclipseUnitTools::UnitSystem RifDerivedEnsembleReader::unitSystem() const
 {
     return RiaEclipseUnitTools::UNITS_UNKNOWN;
 }
-
-//--------------------------------------------------------------------------------------------------
-///
-//--------------------------------------------------------------------------------------------------
-void RifDerivedEnsembleReader::updateData( RimSummaryCase* sumCase1, RimSummaryCase* sumCase2 )
-{
-    m_allErrorAddresses.clear();
-    m_allResultAddresses.clear();
-
-    if ( sumCase1 && sumCase1->summaryReader() )
-    {
-        m_allErrorAddresses  = sumCase1->summaryReader()->allErrorAddresses();
-        m_allResultAddresses = sumCase1->summaryReader()->allResultAddresses();
-    }
-}
diff --git a/ApplicationCode/FileInterface/RifDerivedEnsembleReader.h b/ApplicationCode/FileInterface/RifDerivedEnsembleReader.h
index 316e4d5310..89eef093d8 100644
--- a/ApplicationCode/FileInterface/RifDerivedEnsembleReader.h
+++ b/ApplicationCode/FileInterface/RifDerivedEnsembleReader.h
@@ -30,15 +30,15 @@ class RimSummaryCase;
 class RifDerivedEnsembleReader : public RifSummaryReaderInterface
 {
 public:
-    RifDerivedEnsembleReader( RimDerivedSummaryCase* derivedCase, RifSummaryReaderInterface* sourceSummaryReader1 );
+    RifDerivedEnsembleReader( RimDerivedSummaryCase*     derivedCase,
+                              RifSummaryReaderInterface* sourceSummaryReader1,
+                              RifSummaryReaderInterface* sourceSummaryReader2 );
 
     const std::vector<time_t>& timeSteps( const RifEclipseSummaryAddress& resultAddress ) const override;
     bool        values( const RifEclipseSummaryAddress& resultAddress, std::vector<double>* values ) const override;
     std::string unitName( const RifEclipseSummaryAddress& resultAddress ) const override;
     RiaEclipseUnitTools::UnitSystem unitSystem() const override;
 
-    void updateData( RimSummaryCase* sumCase1, RimSummaryCase* sumCase2 );
-
 private:
     RimDerivedSummaryCase* m_derivedCase;
 };
diff --git a/ApplicationCode/ProjectDataModel/Summary/RimDerivedSummaryCase.cpp b/ApplicationCode/ProjectDataModel/Summary/RimDerivedSummaryCase.cpp
index 25228a4f61..72f56965c9 100644
--- a/ApplicationCode/ProjectDataModel/Summary/RimDerivedSummaryCase.cpp
+++ b/ApplicationCode/ProjectDataModel/Summary/RimDerivedSummaryCase.cpp
@@ -235,7 +235,8 @@ QString RimDerivedSummaryCase::caseName() const
 //--------------------------------------------------------------------------------------------------
 void RimDerivedSummaryCase::createSummaryReaderInterface()
 {
-    RifSummaryReaderInterface* summaryCase1Reader = nullptr;
+    RifSummaryReaderInterface* summaryCase1Reader1 = nullptr;
+    RifSummaryReaderInterface* summaryCase1Reader2 = nullptr;
     if ( m_summaryCase1 )
     {
         if ( !m_summaryCase1->summaryReader() )
@@ -243,10 +244,19 @@ void RimDerivedSummaryCase::createSummaryReaderInterface()
             m_summaryCase1->createSummaryReaderInterface();
         }
 
-        summaryCase1Reader = m_summaryCase1->summaryReader();
+        summaryCase1Reader1 = m_summaryCase1->summaryReader();
+    }
+    if ( m_summaryCase2 )
+    {
+        if ( !m_summaryCase2->summaryReader() )
+        {
+            m_summaryCase2->createSummaryReaderInterface();
+        }
+
+        summaryCase1Reader2 = m_summaryCase2->summaryReader();
     }
 
-    m_reader.reset( new RifDerivedEnsembleReader( this, summaryCase1Reader ) );
+    m_reader.reset( new RifDerivedEnsembleReader( this, summaryCase1Reader1, summaryCase1Reader2 ) );
 }
 
 //--------------------------------------------------------------------------------------------------
@@ -329,15 +339,7 @@ void RimDerivedSummaryCase::fieldChangedByUi( const caf::PdmFieldHandle* changed
     bool reloadData = false;
     if ( changedField == &m_summaryCase2 || changedField == &m_summaryCase1 )
     {
-        if ( !m_reader )
-        {
-            createSummaryReaderInterface();
-        }
-
-        if ( m_reader )
-        {
-            m_reader->updateData( m_summaryCase1(), m_summaryCase2() );
-        }
+        createSummaryReaderInterface();
 
         reloadData = true;
     }