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).