Two-string sweep picking
Two-string sweep picking: instead of playing each note with a separate up-and-down stroke as in classic pick playing, this exercise sweeps the D and G strings in a single continuous downward motion, placing the fretting fingers one after another as the pick moves across. The difficulty isn't the pick's speed, which is set purely by the wrist motion, but the exact synchronisation between that motion and placing each finger at the right moment.
Tips
Separate the two motions first: sweep the two strings without fretting anything, then fret the notes without sweeping, before combining them.
Slow down until you can hear each note distinctly one after the other: if the two notes blur together or one is missing, the left-hand/right-hand sync isn't exact yet.
Lightly mute the string above with the edge of the palm: that's what stops it still ringing while the pick is already attacking the next one.