I was in the same boat as you, one Pro 3 wasn't able to maintain my tank. I ended up using it on my 75 gal and got a 2262 for my large tank. I've not been happy with my Pro 3, but I am glad they work for you.
I really had no problem with one, Eheim makes awesome filters. They bred like crazy with just one. They are getting to the size now where I just felt more comfortable having that extra amount and the ease of cleaning of the 2180 cant be beat. With 2 filters i can probably just clean once a year now