Ok to use one of the two 12V batteries of a 24V system?
That's probably a bad idea.

Hooking your... IT stuff to a single battery will discharge it faster than the other, especially if you're planning on using your devices when the engine is off.

If your devices power consumption is low compared to the battery size then you are good to go, but a 24V->12V DCDC converter (that's what you meant, right?) is still the best solution.

If you have multiple devices and you can balance the load between the two batteries that's fair enough too but keep in mind that your devices will not be sharing the same ground, so connecting them together would probably result in harmful (but spectacular) failures. Like batteries exploding failures.