Many civilised countries explicitely forbid this -- you only execute the highest punishment you are given if the accusations are related. So if you are accused of money laundering and get 8 years for that, and of information theft (which helped you do the money launering) and you get 5 years for that, the final punishment is 8 years -- because, arguably, what you're guilty of is money laundering, information theft or not.
The problem is how do we make migrating to such a scheme politically tenable? Right now we are in a situation where politicians dream up new "multiplier" laws (for instance, throwing on extra punishment or at least higher minimum punishments for committing a crime while wearing a mask) so that they can be seen as "tough on crime". We aren't just coasting in the wrong direction, we are making an effort to go in the wrong direction.
It seems hopeless to me. The general population doesn't give a shit, they are bloodthirsty and think of "criminals" as some distinct segment of society that they don't need to empathize with (hell, we can't even get executions widely abolished, a far more egregious issue...) So long as that is the case the politicians will keep on making the situation even worse for personal gain.
Under the U.S. sentencing guidelines (e.g. http://www.ussc.gov/Guidelines/2011_Guidelines/Manual_HTML/5... or state equivalents), this is already the case. Although you may be sentenced to "105 years" for multiple crimes, if they're related to the same activity, those sentences are usually served "concurrently." It's left to the judge's discretion.
Your complaint at this point should be with prosecutors that use the most draconian possible sentence to promote themselves and pressure defendants, and with a media that loves the shock value of numbers like "105 years" when such a sentence would almost certainly never happen.
Without the laws to enable it, the prosecutors would be unable to do that, and the only way to prevent prosecutors from doing that is to make laws that forbid it. The only way to change this while working within the system is through the legislators.
One way is to highlight cases with generally good people getting destroyed by overzealous prosecutors. Its unfortunate, but this is one of those issues where optics is huge. If you're black or hispanic or have tattoos, many people will assume "oh, well he probably committed another crime anyway, so its okay to go overboard on this one".
In holland, if bank robbery and manslaughter would give you 10 years each, then combining those two at the same time would probably give you something like 12 years. Moreover, if you are doing these crimes apart while not being caught in between, you might get a 'combined' sentencing of 14 years.
Oh well I'm happy that you chose this example, because I happen to have a Dutch law degree, and if you're saying this to support the GP who said that "Many civilised countries explicitly forbid this" (cumulative sentencing / consecutive time), then you're wrong (as is the GP).
First, to be clear, there are two cases of committing crimes at the same time: one is when a certain act by necessity implies committing another, lesser offense (the textbook example here is raping somebody on the town square - this almost certainly qualifies as public indecency too, but you'll only be sentenced for the rape). This is called 'eendaadse samenloop' (roughly translated, 'single-act concurrency'). This is not what we are talking about.
What we are talking about is 'meerdaadse samenloop' ('multiple act concurrency'), the rules for which are set in art 58 Sr. (the Dutch criminal code) and basically say this (if lawyers weren't numerically illiterate - sometimes I get the feeling admission to the bar requires a selective lobotomy of those parts of the brain that work with numbers, but I digress -):
Let s be the final sentence, t1 and t2 the effective sentences for each of the individual criminal acts, and t1' and t2' the maximum sentences for those respective criminal acts:
s = min(t1 + t2, max(t1', t2') * 4/3)
In plain words: the prosecutor will charge the suspect with the separate crimes, and ask for separate penalties. The judge will sentence each criminal act separately, and then give the perpetrator a combined jail time for all the criminal acts which is maximized at 30% over the theoretical maximum for the punishment with the longest jail time.
So, in your hypothetical example, the maximum sentence for manslaughter is 15 years (art 287 Sr), for robbery resulting in death also 15 years (art 321(3) Sr). The typical sentence for manslaughter for a first-time offender would be something like 8-10 years; for the other the same. The maximum is 15 * 4 / 3 = 20 years; if the judge sentences the robber to 10 years for each, he will serve 20 years. Note that these are not two separate sentences that are served consecutively, but one sentence that takes into account both acts; but that's merely a semantic difference.
If you still don't believe me, let me quote from 'Een inleiding in het strafrecht in 13 hoofdstukken', Stolwijk (2009) p243, the relevant page of which is also available via Google Books:
"In de praktijk is het matigende effect van de samenloopregeling gering. In de strafzaak zelf wordt een werkelijk plafond in de straftoemeting niet bereikt omdat de rechter ook bij samenloop nimmer aan dat maximum toekomt. De vraag naar meerdaadse of eendaadse samenloop is daarom louter een kwestie van een juiste kwalificatie.'. (summary translation: the effect of the ceiling on combining sentences is very small because that ceiling is seldomly reached anyway).
Sorry for the late reply :-). Canada is the first that springs to mind, and there is at least one European country (from the ex-Communist block no less -- Romania -- that does the same thing under certain circumstances).
