when playing a thing like this i normally sort of "barre" the two strings with my pinky and make a slight tilting movenemt when i change the strings, so it wont sound like a chord. sort of like fretting the B on the 12th string with the tip of my pinky while keeping it arched and just barely touching the e-string, and then i press the e on the 12th fret with the joint-part of my pinky while muting the b-sting with either my right hand palm or the tip of my left hand pinky.