I've never tried it in a 30, but did try many pairs in a 55, IMO and IME a 55 is not enough tank for 2 o's, and even one is borderline. I always ended up selling my Oscars when they grew up because the 55 just wasn'r enough. The problems you will encounter are these.
#1. O's grow about an inch per month give or take 2 weeks or 1/4 inch. in one year a healthy O will be a foot long. at that time he can't turn around in a 12 inch tank without bending.
#2. big messy fish create a lot of waste. O's are the poster child of big messy fish. You will need huge amounts of filtration, and water changes virtually every day to keep the tank clean even when the fish are anly 3 inches long.
#3. Oscars are highly succeptable to HITH (hole in the head disease) although there hasn't yet been a proven cause, a couple of the highly suspected cause are long term exposure to high nitrates (50 ppm or above) and nutrition. nitrition can be achieved in any size tank. Nitrates see #2.
#4. Although Oscars really aren't a highly aggressive fish, tight spaces ond or poor water quality tends to make most fish grumpy IME. and therefore they may get fiesty with each other as they grow.
Draw your own conclusions, but I'd reccomend something in the range of 75 as a bare minimum for two Oscar's.
