Is the grounding of the breadboard common used?
You can use your breadboard to make any connections you like, regardless of its labelling, with the stipulation that you should not connect in this way anything that you would not connect in another way.  It would be prudent to pop the back off your breadboard and take a look so that you know how it works and how it's connected.  If you're not immaculate, when something is going wrong with a breadboarded project, you may want to do the same thing to make sure there are no broken wires or loose bits of metal inside.  You can look at any labels on the breadboard, or any you add, as suggestions or markings to help you keep track of things, but not requirements.

So this means connecting multiple power supplies is actually just its own issue, completely separate from the breadboard.  You should be aware of the difference between an isolated and non-isolated power supply.  Depending on the polarity and how you want to connect your power supplies, you may need your power supplies to be isolated.  A non-isolated power supply has one or more input lines(typically ground) passed directly through to the output of the supply. 

If a power supply has isolation (typically a transformer), you can tie one side of it's output to a side of the output of another power supply to give their voltages a relationship to eachother.  If you have a +5V and a +8V DC isolated supply and you connect their - terminals together, you will have the two separate voltages available to you and you will also be able to measure 0V between their - terminals and 3V between their positive terminals.  If you connect the - terminal of the +5V to the + terminal of the +8V supply, you could consider the point they are joined to be 0V and you would have +5V and -8V available to you, or 13V across the set.  Out of a set of two or more isolated power supplies connected in this way, one may be non isolated without fear of failure.

If power supplies are non isolated, they can still be used together in some situations, with the stipulation that whatever electrical line passes directly through the power supply is the only line that is shared between them.  Bear this in mind if you are dealing with devices with AC inputs.  You could have two non-isolated AC to DC converters.  Both pass neutral through and provide voltage in reference to it.  One gives +15V and one gives -15V.  You could connect them in parallel to eachother with the - of the +15V connected to the + of the -15V to give you +/-15V or 30V end to end.

Keep in mind though:

Many AC devices unto themselves will function just fine if the Line and Neutral connections are reversed, however, if this were done with only one of two non isolated DC power supplies, or if you incorrectly connect the outputs, you will create an AC dead short at the input voltage, because you have directly connected line to neutral.

So learn the difference between isolated and non-isolated PSUs and use them appropriately.