****----There has to be a solution----****

[overhill]

This may not be the most efficient but it should work. It assumes that you have lstMaster, lstSecondary, lstMerged, and a command button to merge them.

Private Sub Command1_Click()
    lstMerged.Clear
    Dim x As Integer
    For x = 0 To lstMaster.ListCount - 1
        '-add all of lstMaster to merged list
        lstMerged.AddItem lstMaster.List(x)
    Next x
    
    Dim y As Integer
    Dim Already As Boolean
    For x = 0 To lstSecondary.ListCount - 1
        '-Check each entry in lstSecondary vs all of merged list
        For y = 0 To lstMerged.ListCount - 1
            If lstMerged.List(y) = lstSecondary.List(x) Then
                Already = True
            End If
        Next y
        If Already = False Then
            lstMerged.AddItem lstSecondary.List(x)
        End If
        Already = False
    Next x
End Sub