As long as both listbox have the same number of items:
Dim myList As New List(Of String)
Dim total As Integer = listbox1.Items.Count -1
For i As Integer = 0 To total
myList.Add(listbox1.Items(i).ToString))
myList.Add(listbox2.Items(i).ToString))
Next
For Each str As String In myList
listbox3.Items.Add(str)
Next
listbox1.Items.Clear()
listbox2.Items.Clear()