From what we know--
--and can be accessed in the B12 thread here, it would be more accurate to say that other supplements should not be taken with B12--not at the same time--as they may interfere with the absorption of the B12.
The cobalamin (B12) molecule is the largest molecule regularly used by the human body, so it is rather easy to interfere with its absorption in the digestive tract. Fiber can bind to it and complex it out, other nutrients compete with at the level of the intestines, etc. Fortunately, we do not need a huge amount of it for our everyday needs, but those who are looking to get more absorbed, especially from oral megadosing during which only 1-2% is likely to be passively absorbed beyond the stomach, should take it apart from anything else. Generally, I've held to at least one hour before consuming anything else, or minimally two hours after.