When two advance, I typically target the second place player and ignore the leader if he has a big lead. I also may try to just be ahead of ANY other player and then work my way up the "leader board". Once in sight, I will focus on 2nd place. I find this makes it easier for me to keep track the OP stacks and figure out my line of attack.
I'm not good at chip tracking yet. I believe weak stack tracking cost me a seat at the FT once when I lost track of an OP who won the last FT seat over me. He beat me by a chip or two! This was one-advance at that point.
Off subject a bit, this OP obscured his stack and even had a dirty stack at one point. I think I called him on it and he acted like he didn't even know he had the chip...it was a large value. He also had a "girlfriend" hanging on him. I've wondered about compatriots slipping a player chips during a tournament to boost their final count. If small enough, it could go unnoticed. The friend was also talking up her man as the greatest ever. I have to admit, it distracted me a bit before things started.
I've also avoided playing sheriff if I am sure of the OPs chip stack or at least my position versus his. Why should I alert the OPs about another player's chip stack?