Copying Data From One Range to Another By Averaging the Intersection

[snb]

@Rick
Although the OP did't ask for any flexibility, I think it's the most flexible one:

Sub M_snb()
  Range("C9:F10").Name = "snb1"
  Range("I9:L10").Name = "snb2"
  
  [snb1] = [if(snb1*snb2>0,int((snb1+snb2)/2),if(snb1&snb2="","",snb1+snb2))]
End Sub

If you 'abhorr' square brackets, use

Range("snb1") = Evaluate("if(snb1*snb2>0,int((snb1+snb2)/2),if(snb1&snb2="""","""",snb1+snb2))")

@Steve

Avoid activate in VBA.
The use of 'Let' is redundant in VBA.
I tried to incorporate my suggestion into your snippet:

Sub Freeze()
    Dim Tt As Long
    Dim Rb As Integer
    Tt = Application.Range("WHERE15").Value
    Rb = Application.Range("reachBACK").Offset(0, -1).Value
    
    If Rb > Tt - 3 Then Rb = Tt - 4
    If Rb <> 0 Then
        Application.ScreenUpdating = False
        Application.Calculation = xlCalculationManual
        
        With Sheets("INVESTOR INTERFACE")
            .Cells(Tt - Rb, 128).Resize(Rb, 3).Name = "snb1"
            .Range("snb1").Offset(, 13).Name = "snb2"
            [snb1] = [if(snb1*snb2>0,int((snb1+snb2)/2),if(snb1&snb2="","",snb1+snb2))]
            If Tt > 12 Then .Range("snb1").Offset(-11, 0).Resize(24, 3).ClearContents
        End With
        Application.Calculation = xlCalculationAutomatic
    End If
End Sub