The only thing to beware of is that your 24V source and 12V source must be able to share a ground.
If they're both coming out of the same PSU, then you should be fine. If you're using two PSU's, there is a fairly easy check you can do:
An easy way to test this is to get a 100-500mA fuse and use the fuse to connect the ground on your 24V supply to the ground on your 12V supply.
If the fuse blows when both PSU's are on, neither of your DC grounds are floating grounds and you're going to have an issue. If the fuse stays intact, then you should be fine.
The fuse will blow if there is a current between the PSU's, which indicates the grounds are not independent of each other.
Note: Most PSU's have a "floating ground" in that the DC ground pin isn't connected to anything (like AC ground), so there should be no issue. Some PSU's can have the DC hooked to the AC ground somehow (sometimes it's DC ground, sometimes it's a set voltage - neither is good practice IMO). As long as at least ONE PSU has a floating ground, or both PSU's have the same DC hookup to ground, then there should not be an issue.
PS: I suggest using a fuse instead of a multimeter set to the AMPs range. It's possible this could draw a few amps. A 100mA fuse gets the job done, and ripping apart a multimeter to replace a fuse and/or buying new probes because the ends have arc burns is usually never fun. It's also dead easy to tell if the fuse is OK or has blown as it's only one component.
