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

While it's never been shown in the series, I imagine tu 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 tu already sold it once, and it's no longer in your possession to do so.

-ahem- well, technecialy tu 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. tu only have one soul so tu can only bring one other soul to hell

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