I will revise the brunette and the brush
I just assumed that, like AM, she was learning it in the field. It's a plausible handling if that is the case. They're not experts (or at least, AM isn't, and I assume the other isn't). You wouldn't expect expert plays from people untrained.
If the brunette is trained, the thing to remember is that you want as few eyes drawn to the participants as possible at any stage of the operation. It's not just the brush that is important. During the approach, if you draw attention, people are more likely to (a) be more aware of you when the brush happens and (b) more likely to remember you after the event. People trained in this sort of thing can recall literally hundreds of people a day, even if just with a glance of a second, sufficiently well to provide a fairly detailed description, and that can come back and bite you later on.
During the brush, above all else, you don't want to draw attention. Speaking when you're in a quiet place makes people look out of curiosity. And in the exit phase, again, you want to minimise attention. A change of direction or an abrupt movement attracts the eye of someone nearby, and the last thing you want is some awkward blighter thinking: "Hang on, didn't he only just come into the room?"
Sometimes, if you're fairly sure that someone is keeping an eye out for a brush, you'll have someone else run interference. A distraction to draw the eye away from you, or something. It doesn't even need to be a knowing accomplice. If you time it right, you can wait until a natural distraction occurs (a couple having a blazing row, for example, or a baby screaming. Babies are really good distractions, and can generally be relied upon to cause a distraction of their own volition) and then time the brush when people are looking elsewhere.
The person to be concerned about is the person in possession. They are the vulnerable one. If there's a glitch, the one not in possession is the one who takes the lead is sorting out the exit strategy for both parties, which generally involves arranging a distraction. In the case you described, with two young ladies, the obvious distraction (and probably too obvious, but they're new to the game) would be to accuse a man of an improper suggestion and then stalk haughtily off. The other will have more cover.
Crowds are better for a brush than empty rooms, for obvious reasons. If there's only two people in a room, any onlooker hasn't got any choices to make about who to look at. With a lot of people and a lot of movement, you've got a lot of distractions.
There's a whole bunch more esoteric stuff, but the basic premise is Don't Be Seen. Oh, and make sure you know your exit strategy at every step of the way.
