It is possible that your code was wrong.
I wrote a simple program, I added a SplitContainer and added a combo box to panel1 of that control. I added 2 panels to the right side.
I made sure they overlapped WITHOUT putting on inside the other, which you may have accidentally done (I accidentally did it and had to separate them). I pre-populated some values into the combo box so I had choices to select from. I changed the back color on my Panels so I could see if they were coming to front or not.
Here is the resulting code:
Public Class Form1
'Every time you select a new item on Combobox1, it will call this code
Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged
Select Case ComboBox1.SelectedIndex
Case 1 'This would be the SECOND ITEM IN THE LIST
Case 2 'This would be the THIRD ITEM IN THE LIST