easy .... Combine 2 Worksheet_Change Events

This is how I would do that:

Option Explicit

Private Sub Worksheet_Change(ByVal Target As Range)
Dim cell As Range

For Each cell In Target

    Select Case cell.Address
        Case Is = "$E$84"
            If cell.Value > Range("C11").Value Then MsgBox "The array area exceeds the available roof space", vbCritical, "Warning!"
        
        Case Is = "$C$9"
            If cell.Value = "Other" Then SmallUtilityData.Show
    
        End Select
Next cell
End Sub

Small typo fixed above.

Only one cell at a time is being checked so it should trigger the first section of code if cell E84 is the one that is changed. The code looks correct, I can't advise more without being able to test it on your workbook.

What's the formula in E84?

Does it contain references to any cells that are being changed manually?

The EVENT you may want to switch to then, is Worksheet_Calculate() event. The only problem with that is the things you want to "pop up" on the two watched cells could BOTH happen every time, it might get weird.

Private Sub Worksheet_Change(ByVal Target As Range)
Dim cell As Range

For Each cell In Target
    Select Case cell.Address
        Case Is = "$C$9"
            If cell.Value = "Other" Then SmallUtilityData.Show
        End Select
Next cell
End Sub

Private Sub Worksheet_Calculate()

If Range("$E$84").Value > Range("C11").Value Then
    MsgBox "The array area exceeds the available roof space", vbCritical, "Warning!"
End If

End Sub

Very smart! If that takes care of your original question, please select Thread Tools from the menu link above and mark this thread as SOLVED. Thanks.