When someone is convicted and sentenced to prison and is ordered by the judge to serve two life sentences it means that the prisoner must serve at least one life sentence before being eligible for parole if they're serving life with a chance of parole.
Then if that parole is denied they would likely have to serve another life sentence before being eligible again for parole.
In most jurisdictions a prisoner has to serve 25 years on a life sentence before being eligible for parole so the prisoner would have to serve another 25 years on their life sentence if they're not granted parole the first time around.
So it would mean that they would have to serve 50 years before they could be released on parole but there's still no guarantee that parole will be granted.
If the person is serving a life sentence without parole the two life sentences is used as insurance to guarantee that the prisoner can never be free again due to appeals.