Option Explicit

Public Sub Test()
  
  Dim folder1 As Outlook.Folder
  Dim folder2 As Outlook.Folder
  
  Set folder1 = ...
  Set folder2 = ...
  
  Dim colAllMailItems As VBA.Collection
  Call AddMailItemsFromFolder(folder1, colAllMailItems)
  Call AddMailItemsFromFolder(folder2, colAllMailItems)
  
  Dim objMailItem As Outlook.MailItem
  For Each objMailItem In colAllMailItems
    ' folder-name, mail-subject
    Debug.Print objMailItem.Parent.Name, objMailItem.Subject
  Next
  
End Sub

Public Sub AddMailItemsFromFolder(ByVal Folder As Outlook.Folder, ByRef MailItemCollection As VBA.Collection)
  
  If MailItemCollection Is Nothing Then
    Set MailItemCollection = New VBA.Collection
  End If
  
  Dim objItem As Object
  For Each objItem In Folder.Items
    If TypeOf objItem Is Outlook.MailItem Then
      Call MailItemCollection.Add(objItem)
    End If
  Next
  
End Sub