Can I Add More Cirax To My Filter?

You can add another cirax box if you want to, there's nothing wrong with doing that.

There will come a point when adding more media is of no extra benefit. The amount of fish in a tank can only feed a certain number of bacteria. These bacteria will spread out into all the available media. If you had an under-rated filter in a very overstocked tank there wouldn't be enough media to house the amount of bacteria the fish needed to cope with their waste. In this case, more media would help, as would a bigger flow rate as an underpowered filter wouldn't get the ammonia and nitrite laden water into the media fast enough to stop the level building up.
If there is already enough media to house the bacteria needed by the fish, adding more will just thin out the density of bacteria in any given bit of media as the bacteria migrate into the new media.