You have to become a VIP member to be able to change your user_name.
But you have to know that the VIP membership costs 50
BTC.
Im not sure you can have two accounts here, probably not.
No, it is allowed and not against forum rules.
edit:
Q: I want to change my forum username. What should I do?
A: You can either donate 50 BTC to become a VIP or PM theymos asking to change your username (although he rarely accepts such requests nowadays).
I don't have a problem with alt accounts as long as they're not used for evading bans.