Advantage of having one valve + crossover hose: You only have to add air to one place and both forks always have the same pressure. (Call this laziness or, if you are feeling charitable, convenience.)
Disadvantage to having the above mentioned setup: If a fork seal fails you loose pressure in both forks. (Call this dangerous.)
I hit a pothole and blew a fork seal on my GL500 and had to drive it home with virtually no front suspension. This was not the most pleasant experience.
Since then every bike I have owned has had separate valves on each fork and no crossover hose, whether it came that way or I had to convert it. As a side benefit, you don't have to remove the hose to remove one fork leg.
It is not as big a deal as you might think if the forks do not have exactly the same air pressure - some bikes come from the factory with different springs left & right and even different shocks (eg. GL1500 has one non-adjustable coil over shock and one air shock).
It is even better if you go through the page linked below and make proper spacers for your forks so that they either do not need air or need a minimal amount of air.
http://www.calsci.com/motorcycleinfo/Suspension2.html