That's happening; holland takes on Belgian prisoners. Or at least they did, then did not, then did, I'm not sure where it stands right now. But definitely happening :)
Belgium has an abysmal track record when it comes to prisons. It's been repeatedly convicted for treating prisoners like cattle, for putting psychiatric patients in regular prisons without treatment etc.
Sending some prisoners to Dutch prisons (including a prison boat, iirc) is one of the best things that's happened to the Belgian penal system in years. Sadly it's only a temporary relief, and it's been cancelled (and resumed) several times.