In my experience, one of the problems with the way some people cross train is that they try to practice arts which are very different or even opposing each other in basic philosophy, principles, and strategy.
High level teachers often will tell you the proverb of chasing two rabbits because you are not really learning past a certain point in either art.
At some point the budoka should become the embodiment of the art. Their intuitive, creative decision making should reflect the essence of the art. If you have conflicting training, then you must stop (for however briefly) and decide which action to take. This means you'll never reach high level in both.
Of course if you're not training with a teacher of this level then the chance you'll get there on your own is very slim. There's also the difficulty in training at a high level with different teachers. They like to have total commitment from students who are likely to progress to the higher levels. Understandable.
I have seen many people get to shodan or nidan in two or three arts and think that adds up to the sum of all the dan ranks. T'aint so! However, if you have reached yondan or higher in one art, it is possible to transfer much of the higher level knowledge to another art. However, I think it's still better to train in arts which have similar principles.
[Edited by Chuck Clark on August 23, 2000 at 07:57pm]