For example: If I trust member A & B and they already are on the DT list their feedback will be shown to me, so then its no reason for me to add them to my specific trust list. Right?
This is correct if you didn't remove DefaultTrust on your trustlist since you automatically trust all DT by default on your trustlist unless you distrust someone on DT then their feedback will not be visible to you by default as trusted feedback.
..OR should I add them to my trust list anyway because two people that I think have good judgment will get more likely to be in the DT list? (Since I seen you can get removed or added on the DT list all the time)
Adding them is more approriate if you really trust someone judgement. If you only trust that two people alone then you should just only list them on your trustlist and remove the defaultTrust.
Answering the title, Yes we are the one who's gonna decide to become a DT since it's decentralized. Theymos can't moderate DT system.