Fish dude is right - you should keep it an extra month to have the beneficial bacteria colonize your other filter. All that BB should be on your gravel and deco as well, but this just helps make sure that you don't lose a huge population of them all at once. Beneficial bacteria eats the bad stuff.
Here's a tip - you will likely have an issue with too much water flow with two filters going in a 10 gallon - while I'm not sure about the Aqua Clear 20, my AC
50 has a flow dial on it where you can regulate the flow strength of the water. I'd turn that down all the way when you have both filters running.