Can anyone thnk of a down side to this? I already have one that does a decent job, but I could still use more filtration ( I think). My water parameters are great but I figured why not if I have the extra room in my sump?

Possible downsides would be more electricity consumption more pumps means more heat. IMO you could always try it and see if it does any good.

The only downsides I can think of would be extra electricity cost and added heat from the pump. The added heat could actually be beneficial this time of year.

I read somewhere a long time ago why 2 skimmers don't work as efficiently as one... the reasoning made sense but that doesn't mean it is true.

Personally, I'd sell the existing one to buy a bigger one. Two is two cups to clean, two pumps using electricity/generating heat/someday needing replacement. Also, if you ever use a controller you will find yourself using two outlets, assuming you turn the skimmer off during some operations.

Twice THE NOISE, MICRO BUBBLES AND MAINTENANCE

Edit: Sorry for the caps.....

I would also think that it would be hard to get the two adjusted EXACTLY the same. If they aren't then one will be skimming heavier than the other, which would seem to be wasting some of the ability of the other one.

I'm not sure what the difference would be in running two skimmers and running a skimmer with two pumps? Maybe dwell time.

I would also think that it would be hard to get the two adjusted EXACTLY the same. If they aren't then one will be skimming heavier than the other, which would seem to be wasting some of the ability of the other one.

That was one of the reasons I thought two might be a good idea. My reasoning was that since I do not have an ato athe water level in my sump fluctuates with evaporation. I could have one set to operate at maximum efficiency at the lower water level and one set to operate with maximum efficiency when the water is high after top offs. I have coralife now that is very temperamental in terms of the water depth of the pump. The deeper the pump is submergeded the less air it pulls in and skimming is poor right after water changes and top offs. I'm not sure if all skimmers have this issue since i have only used this one model

Just before Julian Sprung founded his own company (and before he was into plenum filters), he was advising skimmer-only filtration or filtration dominated by skimmers. And these set-ups frequently included 2 or more skimmers, But that was almost 20 years ago.

If you run an external skimmer you don't have to worry about the water level in the sump. So it may be worth it for you sell yours and buy an external skimmer. Also, with what you're proposing above the skimmer set to run efficiently at the lower level will bubble over when the water in the sump is at the higher level so it'd probably fill the cup and lower the water level in the sump. Which would cause the skimmer set for the high level will never really skim anything into the cup. I'm going with what everybody else says get a bigger skimmer and just run one. You may also just have to suck it up and get an ATO. They're worth every penny.