If you're banned on Bitcointalk, it means you can't use the platform, regardless of whether you have one or more accounts.

And using another account to post anywhere other than for ban appeal is seen as ban evasion.

Regarding your other question, having more than one account is allowed, but the intention behind it matters. Established users may have multiple accounts for different purposes, like side projects or testing or crawling. However, creating multiple accounts for signature campaigns won't result in a ban but will get you tagged.