Here is a list of Power Adaptor Compliance Symbols and their meanings. It's not all inclusive: Standard Certification Marks
The Chinese one is manufactured for Creative, a big US based electronics producer (Creative Zen mp3 player), if the label isn't fake. It is universal 100 to 240 Volts 50/60 Hz, so works in the US and overseas in most cases, with a simple outlet adaptor. It provides up to 1.5 Amps at 5v with a center positive barrel jack.
The german one is generic (No manufacturer labeling or manufactured for), is 230 Volts 50 Hz, so will not work in the U.S. (or other places with 110/60Hz) without a transformer. It only provides 1 Amp at 5v, no labeling to pinout.
FCC approval means that the FCC in the US has tested it and found it produces only a reasonable amount of RF/EMI under very strict conditions, and shouldn't interfere with other electronics around it. Again, this only counts if the label isn't fake, which is a common occurrence.
I'm planning to make a variable split power supply from a 9V DC
adapter.
You are going to be disappointed if you expect more than a few mA from the negative supply and also you will need a buck-boost regulator if you want to create 5V to 12V rail from a single 9V adapter.
Same is true for creating -5V to -12V - you need a much more rigorous approach than either a 555 negative voltage generator or a couple of resistors and capacitors.
How reliable and regulated 9V DC adapters in general?
You can get very reliable ones and can can ones that are really crappy. Best rule here is do some research and pick a reputable supplier.
Best Answer
There is nothing fundamentally wrong with what you propose. Any purpose built dual output power supply is essentially the same thing. So using this approach can be a workable scheme to achieve a dual output + and - rail output.
One thing that must be considered here is that the two power supplies need to have fully floating outputs.
There are a number of serious disadvantages however. Here are some things to consider:
Can the target device powered by this scheme survive safely if one or the other output becomes 0V because one of the wall warts gets unplugged.
A purpose built dual output power supply will have certain fault detection features that will apply equally to both the + and - outputs which will safely shut things down in case of problems. One example would be a primary side safety fuse that blows and disconnects both the outputs from the mains.
Invariably a dual output supply will share some common components and circuits for supporting both outputs. In particular this would be in the mains conversion and isolation circuit. This can lead to a smaller overall design than using two completely separate power supplies.