'Tis a bit silly to sentence someone to a term of incarceration that they have no chance of serving. But 40 years is not long enough in my opinion - they should spend the rest of their lives rotting in a damp, windowless cell.
The convicted should serve the time they are sentenced to. No ifs no buts. I can see no reason why you would let them out after killing two innocent people in a planned act of violence.