Hmm.. well it sounds like the classic case of overstocking... You clean the media only to find that its clogged again in a short time. The problem is that because your bioload is high, even if you clean the media cartridge the growth of the bacteria is exponentially higher than normal because of the available nutrients is higher. Add that to the fact that I'm guessing you're using a stock media cartridge which makes the problem worse since it builds up crud easily with such a small pore size.
There are 2 solutions: 1) lowering the available nutrients in water, 2) modifying/adding filters.
1a) lower nutrients in water by lessening bioload.
You can lower the nutrients by giving away some of your fish. A lighter stocked aquarium will always lead to less (necessary) water changes and fewer filter cleanings.
1b) lower nutrients by adding plants
Plants absorb ammonia readily, which means that the bacteria won't be growing as fast starting from the first phase in the nitrogen cycle. Add a fast growing plant and trim it down and throw it away as it grows. I have amazon frogbit in all my tanks which is easily discarded when it grows too much. You can also build a refugium, but that's a pretty lengthy project for just avoiding cleaning your filters...
1c) lower nutrients by doing more water changes
Self explanatory.
2a) adding more filters
Adding more filters will add more surface area in which the bacteria will grow while keeping the available nutrients the same. The result is that the growth of clogging bacteria in your filters will be less in the later phases. Unfortunately this also means you have another filter to clean, so I'd throw this idea out. Save your money...
2b) Adding a stronger filter
Adding a stronger filter to replace your old ones has very little merit in your situation. Unless the new filter's biological surface area is more than the two filters you have now, you're actually making the situation worse. Save your money. You'll just end up with a filter that clogs up faster. Of course you only have 1 filter to clean...
2c) modifying your filter for more flow
In your case, this will work. You have plenty of power from the two HOB's but your choice of filter media is wrong. The stock media cartridges are way too fine for daily use; its great for polishing water but inefficient for extended periods. What I did is I bought a cheap 1 foot x 1 foot pond filter sponge then cut a slab the size of your filter volume and replaced that for my stock cartridge. The result was that the water was just as clean, but didn't clog because the media is very porous. It works for all HOB's.
I hope this helps you out. Tinkering with your current filtration system doesn't fix the root of the problem, so I'd start with the 1's first.
