Yes this can work, but its risky. You seem still new to how this bio thing works, but with proper caution it can be done. There is always room for error, you just have to know your making an error lol!! When you do move the ac110 to the 180 and only have one on your 75 you need to CUT DOWN FEEDING. Maybe feed once every two days, or less. That way your single filter on the 75 can grow bacteria accordingly without your tank exploding again. Bio will only grow as much as needed. So say if you have your two ac110s and you take one off, your missing bio. Even though one ac110 can properly take care of the 75 not all your bio media is being used, well because it doesn't have to be. Thats why you need to go slow, and really understand bio media, because it eats and grows. Big feedings, and unstable feedings really upset the bacteria. Get a routine going man, and you'll have it down.
Hey man it takes time to grasb this whole bio media thing. I still learn something new about bio media every day. While it is hardy, tap water kills instantly. Good thing your here and learning man! You have a nice fish, and make sure you keep researching bio media and how it works.
Also it could be faster than 2 weeks for a cycle on the 180. You should know by your test results. Read up on the cycle a little more. I also recomend getting a canister filter for the 180, but the ac110s will work. I just personally like the clean look of a canister hidden below. Also a liquid test kit is a must.