Unless the filter you added was cycled -- from another established tank -- it's the water change that's causing water quality to get better. I'm glad you got some reliable tests -- otherwise, you don't really know what's going on, and so you don't know if there's anything you should be doing, you know? I'd keep doing the water changes. Until you have 0 ammonia, 0 nitrite, and some number of nitrate, you're not in the clear. Even then, I test my established tanks bi-weekly (the weeks between water changes) just to make sure everything's okay. With two filters on the tank, you can clean one one month, and the other the next month. When you clean, in case you don't know, only use the tank water (whatever you've pulled out during your water change) so that you preserve as much biological bacteria as possible.
Your tank still has to cycle, and then both of those filters will be good and established. I'd say give it about a month. Then you can start your filter maintenance -- until then, I'd just worry about doing the water changes and keeping ammonia (and later, nitrite) as low as possible.