Enabling two (or a fixed amount) checked in a checkbox/radio button scenario

Alright, here's the case.

Imagine that there's a rectangle with 4 points on it, naming rodUpLeft, rdoUpRight, rdoDownLeft, rdoDownRight as checkbox

I want to make sure that there's exactly TWO checked box checked AT ALL TIMES

Kind of like how radio button works, except instead of just one button got checked, I want two.

So I've tweaked around and came up with a code as below

    If sender.name = firstChoice Then
        sender.checked = True
    ElseIf Not lastChoice = sender.name Then
        Select Case lastChoice
            Case "rdoUpRight"
                rdoUpRight.Checked = False
            Case "rdoUpLeft"
                rdoUpLeft.Checked = False
            Case "rdoDownRight"
                rdoDownRight.Checked = False
            Case "rdoDownLeft"
                rdoDownLeft.Checked = False
        End Select
        lastChoice = firstChoice
        firstChoice = sender.name
    End If

So my initially idea is to track the buttons that were clicked, and uncheck the old button when a new unchecked button is clicked.

However, it does not stop the user from unchecking already checked checkbox, thus breaking the rest of the code.

A help would be much appreciated.