In the cover two, you don't need extremely fast corners who are going to have to blanket a receiver all the way down the field. A good cover two corner only has to smart, quick, and tackle well. He is only responsible for the wide flats and therefore doesn't have to worry about getting burned deep (unless it's man 2); since he is pretty much scouting the play on his island, he needs to be ready to tackle anything that comes his way. Cover two corners generally make a lot tackles in run support (see Derrick Strait of OU) and aren't considered one-on-one guys (see also Derrick Strait). You have to have athletic, smart, safeties to make it work because they are responsible for one half of the deep field each. They need to able to cover the go, the hitch and go, the corner, the curl, and try not to get exploited by the post. Posts and other routes to the middle of the field take advantage of the safeties' outside position and are the biggest threat to a cover two defense. I love the cover two and here is my minimum requirement for my DB's:
CB: 6'; 190; 75 SPD; 85 ACC; 60 TAK
S: 6'; 200; 85 SPD; 85 ACC; 70 TAK
Here is a link explaining the 3-3-5 defense, courtesy of ADW aka MadBomber:
Learn to Love the 3-3-5