you can argue the cross train either way. one can train in one art for along time to get it right then take on something else. i have seen folks who had difficult picking up new and different stuffs. just look at some of the aikido seminars, how often you see the seminar instructor shown folks stuffs, only to see folks went and did what they have always been done at their own dojo? that's just within aikido alone. adding a different art? good luck on that one.
of course there are those who exposed to multiple arts at the same time earlier on, who don't seem to have problem picking up new and unusual stuffs on the fly, and tend to be quite open minded about things.
martial arts really come down to training how to control your body to make it does what you want it to do. if i want to "take this foot and put on that side of your face, and there is nothing you can do about it" i can without even thinking about it. of course, for some of us, who got a bit older, where our body said "oh hell no! you ain't gonna move my fat ass anywhere!"