From my understanding most instructors seem to frown on cross-training in different styles. Why hasn't been made clear and my only (tongue in cheek) speculation is that they don't want their student's money to go to anyone else. But serious speculation is that it may hinder/alter the student's teachings/perspective of their first art, which ever one that may be.
Yes, that and the fact that many Inst think that their art is the best. If there was 1 best art, then everybody would be doing it! You hear people all the time say "Well, why do this when its already in the art you're studying?" Well, that might be the case, but IMO, if you want to throw better punches, learn from a boxer. If you want to grapple better, train with a grappler.
Of all the different ones that are out there it can be confusing for the novice or beginner. Comments here rightly said that they need to search out what they desire to learn from said chosen art. If merely SD then finding the right one that fits their ideal of SD should be reasonably simple.
If searching for inner enlightenment or self-awareness then finding the right one with the right philosophy can be difficult.
I've studied a bit of many different arts. Though I've no formal schooling in any of them I still call myself a martial artist because of years of training and discipline with each one. When you're a poor man you get what you can whenever-wherever you can get it. It's my only excuse for not having a belt. But I don't feel that it makes me any less of a martial artist because what I have learned has effectively done what it was supposed to. Protect me and enlighten me.
Again, doing more than 1 art is not for everybody and IMO should not be done until the student already has a good base in their first art. Again, considering that there are so many different ones, stopping your art and training in another might not be the best thing. But nobody never said that you can't sample some of the other food out there.
So the question of: is one enough? It depends upon the individual. But I've learned that if one is to apply what they learn out in the real world in regards to SD then learning different styles is important because not all situations will be the same.
If one does not anticipate conflict outside the Dojo or tournament arena then staying with the "Mother-art" is a good idea.
