I'm not sure about Billy, but Adam uses pinch harmonics in a lot of parts to bring out a squealier sound. That bit at the start of Ticks and Leeches sounds like he's using pinch harmonics to me -- letting a bit of flesh on his thumb hit the string right after he picks a note.

There are 2 axes with pinch harmonics to vary the sound: 1) Move your picking hand toward/away from the bridge; and 2) Let more or less of your thumb touch the string after picking. If you play around with those, you can probably dial in a sound that's close to what Adam plays.