I was successful in keeping 2 different pairs of oscars in 2 separate 75 gallon tanks. With the first pair, I had two pictus catfish and they all did just fine together. I wouldn't recommend a pleco in this set up though, because of the copius amounts of waste produced by the oscars alone.
And don't forget, just because it's been done, doesn't mean that the 2 oscars you will end up getting will get along for the rest of their lives. With Nicholas and Armand, there were 2 close calls where Nicholas had suddenly turned on Armand and beat the crud out of him.
You can also cut down on waste by getting them to eat a staple pelleted diet with occassional meaty treats and only feeding every other day.
I used to feed my oscars a super protein diet and believe me, I saw it back out the other end the next day!! :eek:
After switching them all to pellets, I barely ever saw oscar poo again after that.