If Ты Pull The String Once Can Ты Do it Again?

While it's never been shown in the series, I imagine Ты probably can't. Ai has denied some requests, so she'd probably deny one of someone who has already used it once. I think it's like trying to sell your soul, only Ты already sold it once, and it's no longer in your possession to do so.

-ahem- well, technecialy Ты can't really go to hell twice(you know like"to dig a double grave") so I don't think so

I think it's one soul for another. Ты only have one soul so Ты can only bring one other soul to hell

Well, if Ты send someone to hell Ты need to pay of what Ты have done, Ты shall be sent to hell too, but that's after Ты die. Ты only have one soul therefore, Ты can only send a person once. Enma Ai would probably, no she will really deny your request.