Actually, if you are talking about switch mode converters, I see little reason to ever put them in series. The efficiency is lower since you are going through 2 conversions, and the current capacity of the first converter has to be higher than necessary since it has to handle the power at both voltages.
The only time I would do it would be if the input voltage was out of the range of the typical DC to DC converter you can get, say 90V. It would be reasonable to use only one converter handling that high voltage. If they are more like your example, the typical low cost converters you can get will handle the input and I would just use two in parallel.
Bob