Okay, so for some reason, when I was searching for information on 12 V power supplies that you can connect multiple 12 V devices to a single power supply, I found these really great and useful sources, but because they were all about PC power supplies and not the more general topic, I scrapped them and failed to save them the first time through. So, now I go backwards in time to search for what I’ve found previously and note this article down again.
So, the question, can you connect multiple 12 V devices to a single 12 V power supply? In general, yes, so long as your power supply can provide adequate current. PC power supplies in particular may have relatively high current requirements and will use multiple power supply rails with separate current limiters in order to comply with the safety standard IED 60950. Sometimes there may even be separate switching regulators to help with noise isolation. This website also has lots of great information about PC power supplies in general. A PC power supply has 12 V, 5 V, and 3.3 V outputs.
20180512/http://www.playtool.com/pages/psumultirail/multirails.html
20180512/http://www.playtool.com/pages/psurailhistory/rails.html
So, yes, in general, for low current, you can connect multiple 12 V devices to a single 12 V power supply.
Additionally, along the way, I found out that there are many people who convert a PC ATX power supply into a cheap “lab bench” power supply. Originally I found an Instructables article, but in fact this is so popular that there are so many such Instructables articles that I couldn’t find the original.
The original Instructables article can be summarized like this. Lab bench power supplies can be rather expensive, but surely you have loads of old PC ATX power supplies lying around, so why not convert one to get a cheap lab bench power supply? Then the Instructable went into opening up the power supply, making some internal modifications so that multiple voltage outputs could be provided, adding a neon light since switched-mode power supplies cannot operate without a load across the output and can get damaged if such a condition occurs, and provided a variable resistor dial at the front to adjust the output voltage.
But, for the sake of completeness, I link somewhat similar articles here. Alas, it is still a far cry from the original source that I found.
20191204/https://www.instructables.com/id/Hack-an-Old-Pc-Power-Supply-Into-a-Lab-Bench-Power/
20191204/https://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supply.html
20191204/https://www.instructables.com/id/Encyclopedia-of-ATX-to-Bench-Power-Supply-Conversi/