Based on my 5 months of experience having a 3-4" rhom and 5.5" irritan divided in my 75 gallon, I dont recommend it. I previously had just the irritan, and he was very active and aggressive. When I split the tank, the irritan more or less sat around. The small rhom swam around some, but the situation just didnt satisfy me or my fish(im guessing). I sold the rhom about 3 weeks ago, and the irritan is right back the way he was before, being active and very aggressive.
If you still want to divide your tank, I recommend eggcrate. It is the stuff used for the grate under commercial lights. You can get it at home depot or lowes, and it cuts easily. It isnt too difficult to fit tightly in the aquarium, so no suction cups are needed. It was abrasive on my fish like some other people have said about their experience with it.