If the question is - can a 12 vdc system & a fully independent 24vdc system, with separate charging
systems, share the same negative wire (sometimes called ground)? The answer is yes, if everything is sized correctly.
If the question is - can you assume that these two systems, as wired by someone else, will necessarily share the same negative connection? The answer is no.
If the question is - can I take 12vdc off of one of the two series-wired batteries in my 24vdc system & use it to run a 12vdc device? The answer is only for a short period of time in an emergency
situation, as this will cause imbalanced discharge of the two batteries in series & a typical 24 volt charging
system will not correct it.