"One person at a time," confirms Shell Bell. "I don't know if they get sensory feedback from it so they could tell if you were there even invisible."
"...Sherlock, would you mind accompanying me, so that one of us will be able to bail us out no matter what?"
"If I accompany you, it should not be necessary to get close enough to be noticed at all."
"I can inspect objects in the same world from an arbitrary distance away. You could copy the power, but you've never seen Nathaniel with it."
"All right. And Shell Bell, you can hold the door - you'll be okay?"
She shifts a locus of her perception power directly to Voice.
After a moment spent studying their physical structure, she nods.
Angela spends a moment studying the apartment the door opens into - she'd like to have a shot at forcing the door in case she ever needs it - and then she turns around. "Now what do I do with Nathaniel, though?" she murmurs. "Templates are usually well-matched instance to instance - but Nathaniel's a child, he hasn't hurt anyone -"
"It could just be Downside. Downside can change people," murmurs Shell Bell, letting the door close.
"I suppose as long as Nathaniel's seen Milliways and you, now, I can explain to him - in age-appropriate terms - what happened. Warn him."
"Not really, but it seems worse not to tell him. At least in general terms."
"For the same reason I'm planning to ask Juliet just what happened to her in her vaguely described misadventure, when I meet her. It's important to know the pitfalls. Nathaniel's happen to be particularly dire."