I bought the same setup from walmart a few years ago when i started my tank for my newborn baby girl.
Before long, I found myself hooked and decided some of the original parts just werent up to snuff.
For the money, I do not think you can beat an Aquaclear filter. On my 55g I have 2 Aquaclear 500's and experience no problems with too much water movement.
What I like about 2 filters is that if one should go out (which, by the way, in almost 4 years of fish keeping, ive never had a filter go out) I will have another working filter already installed and ready to go.
The other reason is that I keep cichlids, and I like the ability to be able to switch around what type of media I put in the second filter.
During a recent move, I found my nitrates had spiked (im assuming it was due to the stirring of all the gunk in the gravel). I left one filter alone and put nitrate absorbing material in a fliter bag in the other.
Another good reason to get 2 filters, IMO, is to remove dead spots from the water. I like a little current running around my tank and 2 filters seem to ensure that all of the water is moving around at a good rate.
90% of the time though, I keep 2 sponges in the main filter and 1 sponge plus filter floss in the other.
Some people may call this overkill, but during the move I removed and sold an Eheim 2026 that was being used in conjunction with the 2 Aquaclears.
And, just FYI, I experience no ill effects from either running all 3 filters nor did I experience any problems when I completely removed the Eheim and all of its media. I simply removed a filter bag full of old media from the Eheim and dropped it in my aquaclear in place of the filter floss.
I guess, to give you a simple answer, I think it is near impossible to overfiltrate a 55g tank.