Sub test()
Dim dt As Variant, Rmax As Long, RR As Long
Rmax = Range("A65536").End(xlUp).Row
For RR = 2 To Rmax&
Cells(RR, 2).Value = Cells(RR, 2).Value + Cells(RR, 3).Value
Cells(RR, 3).ClearContents '結合時アラート防止のため
Range(Cells(RR, 2), Cells(RR, 3)).Merge '結合
Next
End Sub