multiple listbox selections

I have a listbox in a datagrid that can contain up to two selections. How can I return the selected indexes of the two values? I know how to do it for a listbox that has a selectionmode of "single", but not for multiple.

Here is my HTML:
<asp:TemplateColumn HeaderText="Riders">
<ItemTemplate>
<asp:listbox AutoPostBack="False" BackColor="#ffffff" id="lstRider" runat="server" Rows="1" DataSource="<%# DsRider1 %>" Enabled="True" SelectionMode="Multiple" DataTextField="UserName" DataValueField="UserName" SelectedIndex='<%# GetRiderUserName() %>' />
</asp:listbox>
</ItemTemplate>
</asp:TemplateColumn>


I'm executing a function "GetRiderUserName" in my code-behind to return the selected indices, but don't quite know how to do it. This is as far as I've got:

Public Function GetRiderUserName() As Integer

'Returns the index of the username
Dim i As Integer
Dim blnFound As Boolean = False
Dim bytFoundCnt As Byte = 0

For i = 0 To DsCalDteRider1.CalDteRiders.Rows.Count
If DsCalDteRider1.CalDteRiders.Item(i).Username = DsRider1.Riders.Item(i).Username Then
blnFound = True
If bytFoundCnt = 2 Then 'Max selections allowed
Exit For
End If
Else
blnFound = False
End If
Next

If blnFound Then
Return DsRider1.Riders.Item(i).RowNbr
Else
Return -1
End If

End Function

How can I turn "Return DsRider1.Riders.Item(i).RowNbr" into the selected indexes of a possible two selections?

I figured it out. However, there is one thing I'm confused about though. The following statement I have for my other listbox in the DataGrid1_ItemDataBound event which is a single selection listbox:

lstCrewChiefTemp.SelectedIndex = lstCrewChiefTemp.Items.IndexOf(lstCrewChiefTemp.It ems.FindByValue(e.Item.DataItem("Username")))

The above statement automatically selects the value in the listbox without apparantly comparing it to anything else to see if it matches (like I did in the multiple listbox - following example): Note: the "Username" is the DataValueField for the listbox and I have a datasource associated with it: DsCrewChief1

For i = 0 To DsCalDteRider1.CalDteRiders.Rows.Count
If DsCalDteRider1.CalDteRiders.Item(i).Username = DsRider1.Riders.Item(i).Username Then
strUsername = DsRider1.Riders.Item(i).Username
lstRiderTemp.Items.FindByValue(strUsername).Select ed = True
bytFoundCnt += bytFoundCnt
If bytFoundCnt = 2 Then 'Max selections allowed
Exit For
End If
End If
Next

How does this statement work correctly in selecting the proper listitems???

lstCrewChiefTemp.SelectedIndex = lstCrewChiefTemp.Items.IndexOf(lstCrewChiefTemp.It ems.FindByValue(e.Item.DataItem("Username")))