Same voltage but different potential
The concept you are missing is that a supply can be isolated. This means that (within reason) the output can be referenced in any way you like. If a DC supply is isolated, then you can reference the positive or negative to earth if you like. Or to some other voltage you have handy. It can even be referenced to a moving voltage such as the hot mains voltage (which of course makes it equally hazardous as mains voltage).

Also, if you have two isolated supplies, they can be connected together in series or parallel, as you choose. What is the potential of an isolated supply? In my opinion, it is best to think of it as undefined. You don't know what it is, you can't count on it being any particular value, it may drift with time, and if you try to measure it, you will probably end up changing it.

In general, commercial power supplies will either be referenced to an earth ground potential (supplied by you with a wire when you connect the supply) or isolated. You will not find a power supply that is referenced to some arbitrary voltage such as 90V.

In your specific case, you should be able to tie your grounds together without fear.