Or if you can under some conditions, why is is a bad practice?
Who says you can't or parallel power supplies?
It all depends the power supplies in question.
I used to design bench power supplies for a well known test and measurement company.
All our designs were capable of being connected in series as they had floating outputs you just had to ensure that they were never trying to sink current under any load they were asked to supply and that no terminal was more than 300V away from earth.
You could also parallel them. They would not share but were self protecting for an overload so if you drew more current that one could supply it would current limit and the other would start to conduct. For example you have two 20A supplies and ask for 15A it may all come from a single supply but if you ask for 30A one could be supplying 20A (current limit) and the the other 10A.
You need to check with the manufacturer however, not all power supplies are designed to work this way.
For AC supplies like a floating transformer secondary winding, series connection is sometimes done, taking care to get the phasing correct.
For DC voltage supplies, series connections can be done, provided one is a floating supply. There are a few precautionary checks to be made first:
Can the floating supply withstand the common-mode voltage forced on it by the "bottom" supply?
Are both supplies able to accommodate reverse voltage applied to their outputs? Some supplies add a reverse-protection diode at their output terminals in an effort to prevent reverse voltage from destroying the regulating electronics. This diode must be rated to accept possible over-current resulting from the two-supply connection.
Should either DC supply not be protected from reverse-voltage, turning either/both on-to-off or off-to-on is problematic...it is unlikely that their output voltage will rise (or fall) simultaneously. The momentary surge of reverse current and/or voltage could easily destroy them.