I would suggest that it depends on your level of expertise. If you are relatively new, I would suggest you trade only one market. That's because it takes a long time to 'know' a market; it's intricacies, personality, and how it correlates with other markets, and how it is affected by news, etc. So you find out what works on that market, and trade it.
It is not always true that what applies to one market will automatically apply to another - each is a different animal, and needs to be treated as such.
I learnt on one market, and because I am now so familiar with it, it is quite easy for me to trade other markets. Looking back I would do the same thing again.