Sub test()
Dim arr As Variant, brr As Variant, crr As Variant
arr = Range(Cells(1, 1), Cells(Rows.Count, 1).End(xlUp)).Value
brr = Range(Cells(1, 2), Cells(Rows.Count, 2).End(xlUp)).Value
ReDim crr(UBound(arr) * UBound(brr))
Dim i As Long, j As Long, k As Long
For j = 1 To UBound(arr)
For k = 1 To UBound(brr)
crr(i) = arr(j, 1) & brr(k, 1)
i = i + 1
Next k
Next j
Cells(1, 3).Resize(UBound(crr) - 1).Value = Application.Transpose(crr)