Why would you want a little more umph in your biological filtration? Your tank is still new, and the beneficial bacteria hasn't had enough time to colonize the first filter. You don't know whether or not the biological filtration in the first filter is enough, and whether or not you need to increase the surface area and the population of beneficial bacteria living on it.
Let me phrase that in a different way. The bacteria's job is to convert ammonia into nitrite and then to nitrate. Ammonia is the waste that comes out of fish. It's very toxic, and can kill fish at 2 ppm. Four and your fish have no chance. Nitrite is slightly less toxic than ammonia, and a concentration of 2 ppm of nitrite won't cause as much damage to a fish as that same amount of nitrogen in the form of ammonia would. If the nitrogen is converted to nitrate, then it is even less toxic, and it takes about 30-40 ppm before fish show signs of distress. It's the same nitrogen atom, just in different forms that have different maximum concentrations they can reach before they're toxic. The bacteria put the nitrogen in a less toxic molecule. They don't remove the nitrogen from the water.
If your tank has been set up for a few months and you still have a nonzero concentration of ammonia or nitrite then yes, you need a higher population of nitrosomonas and nitrospira bacteria in order to handle all the conversion from ammonia to nitrite to nitrate. But if your ammonia and nitrite are already at 0 ppm, then there's nothing more that those bacteria can do for you. Increasing the population of beneficial bacteria won't lower your nitrate concentration. You still have to have some way to remove nitrate yourself. You can either grow live plants that remove nitrogen from the water column and use it to form new tissues, or you can perform a water change, which dilutes the concentration of nitrate in the water.
Information about beneficial bacteria: http://www.fishkeeping.co.uk/articles_5 ... rticle.htm
It's possible that the amount of surface area you have in the main filter is plenty of acreage for your colony of beneficial bacteria to live on. Your population of bacteria right now, since your tank isn't yet fully cycled, is very low. Think of the filter as a city with a million homes. Your bacteria are reproducing very slowly, so only one in ten homes has been moved into. And now here you are adding a second city with some second million homes. Why? They're not done moving into the first filter's homes yet. That's probably a pretty stupid analogy and I apologize for it, but my point is, you should wait until your tank is fully cycled and see if you're having problems cycling with only the first filter before you decide that a second filter is necessary.