Hello,
"Other Users" folder, as the name suggests belongs to other users. Therefore, you cannot simply make the fax "appear" in those folder once a fax is sent. In order to copy or move the fax to another user's mailbox, you need to forward or route the fax.
You can have a look at ForwardToUsers and RouteToUser methods in Fax class in order to accomplish this.
I hope this clarifies your question.
Thanks.
Arjuna