@Daniel_Stauffer I’m sure you could find a dual voltage supply, but it would probably be more expensive and overkill vs a single 24V supply + small 24V to 12V regulator, since the 12V supply’s current requirements shouldn’t be that much (just fans and LEDs in my case).
Wiring for 24V/12V systems is a little trickier but not too bad. As mentioned previously all the grounds should tied together. Not a problem if you use the popular LM2596-based switchers (like the one you have pictured).
Most (probably all) boards (I have a Duet Wifi and used to have an Azteeg X3) switch the ground line to turn fans, heaters, etc. on and off. So you would have the board run off of 24V, so your steppers and all your devices that need 24V would be plugged into the board normally. For 12V devices, you want the negative side to go to the negative connector on the board (i.e. normally), but the positive wire should go to the +12V supply instead of the board’s 24V post. I’m lazy so I would just cut the red wire off at the connector and wire it to your 12V source. But you could also make an adapter board so you don’t mess up the fan connector.
The Duet Wifi (and possibly other boards) allow the fan supply voltage on the board to be different than the stepper/hot-end supply. So I don’t need to modify any connectors - I just run the 12V from my regulator into the fan supply voltage input.
Does that all make sense?
@Fred_Hamilton yes this definitely makes sense. So thank y’all for the great input on the topic. I appreciate it alot!
@Daniel_Stauffer I have a printer with a similar setup, and how @Fred_Hamilton described it is pretty much exactly how I set it up (though I use 24V for the bed).
With regards to “letting the smoke out” with one of those modules if the grounds are independent: This isn’t always the case. You may simply see that your output voltage might be screwy. eg: lots of ripple, not the right voltage, not able to produce enough current. Lots of different failure modes and it’ll all depend on the specific design of the module as to what happens. Of course, smoke leaking out is still bad though. 
How did you connect the wires to the converter? I can’t seem to find suitable terminal connectors to solder on. Any source for something similar? I could also try to use a pair of molex connectors and just cut the pins off I don’t need. But I would prefer a better solution.
I also ran a test on one of the converters today. Went pretty well. At least the multimeter didn’t go wild. I would like to connect them to a oscilloscope to see how the voltage behaves. But I don’t have one except of wiring together an arduino poor man’s oscilloscope.
@Stuart_Young You’re totally right that it wouldn’t matter if the input and output were isolated from each other, but a fully isolated switcher’s usually more expensive and isn’t necessary for this app. I had assumed we were talking about someone using a negative regulator (-24V to -12V) without an isolated output thinking they could just connect the regulator’s GND_IN in to +24V and the regulator’s -24V in to GND, in which case the regulator’s outputs would be +24V and +12V - the right polarity and relative difference, but everything shifted up 12V. You see your magic smoke either from powering a 12V device at 24V (if you connect the regulator’s most positive output to your devices thinking it is +12V) or from tying the most negative regulator output (sitting at +12V) to ground to shift the output voltage (as you could safely do with an isolated switcher).
Probably not a likely scenario, but I remember back in the linear regulator days when inexperienced/overly clever people would try to substitute LM7805s with LM7905s (or vice-versa) with exciting unintended consequences.
@Daniel_Stauffer Connecting the 3 (the grounds the same on both sides) wires to the regulator should be straightforward - just solder the wires (hopefully black for ground and red or at least something non-black for the positive voltages) to the 3 holes in the corners of the regulator.
As far as connecting the other side of those wires, you can tie the regulator’s inputs (+24V and ground) to the power supply’s output block that goes to the controller board or where that power supply connects to your controller board, whichever is easier. Then the remaining wire (the 12V output from the regulator) becomes the positive supply for whatever you want to run from 12V. My DuetWifi has a fan voltage input connection, so I just crimped the wire to 1 pin of a 2-pin connector and connected it to the board. If you don’t have something like that on your board, you have to connect the regulator output and +12V device inputs some other way. I’m lazy so I’d probably just solder them together and cover the connection with heatshrink, but you could also use proper male/female connectors.
If your connections are solid and you’re using reasonable size wires (18 gauge or thicker), it should work fine - you shouldn’t need a scope.