VBA Macro Event For Questionnaire With "X" & Yes/No

I have created a worksheet to track/calculate compliance with a number of important items at a number of business locations on a number of questions. This workbook will be posted on a central server that will allow 25+ employees to access it from the field and "double-click" to enter an X in either a "Yes" or "No" column for about 100+ locations (the code also will remove the "X" in either column if another "X" is entered into the other column to prevent double entries). The workbook will be quite elaborate and will be locked down except for those cells unprotected for data entry (identifying location, date, etc.).

Below is the code and it works just fine as written with two columns to chose from to "double-click" and enter an "X" (columns C & D for rows 4 thru 15). I want to "freeze frames" locking columns A & B and allow scrolling to the right to enter each New Location and allow the same code to apply to the same rows but column groupings of E & F, G & H, I & J, ...until I have about 100+ sets of two columns identified in the code.

I have tried a variety of changes to the "Union(Range" and "Intersect(Target, Range" last night and this morning to no avail and have gotten nothing but a headache and a desire to drink heavily from the exercise.

Private Sub Worksheet_BeforeDoubleClick( _
ByVal Target As Range, Cancel As Boolean)
Dim rInt As Range
Dim rCell As Range

Set rInt = Intersect(Target, Range("C4:D15"))
If Not rInt Is Nothing Then
For Each rCell In rInt
rCell.Value = "X"
Next
End If
Set rInt = Nothing
Set rCell = Nothing
Cancel = True