I'm sorry, usernames are wiki-wide. All wikis will see it the same. I mean, you *can* change it, ONCE, but it will always been the same on all wikis.
However, there is nothing wrong with having multiple accounts if having wiki-specific usernames is that important to you. Just make sure you secure each so no one "accidentally" gains access to any.
While some wikis and users have rules calling for automatic blocks for owners of multiple accounts, it is *not* a violation of FANDOM as a whole. Abusing multiple accounts is, but just having them. Many Staff have a few both for work purposes, and personal.
Personally, I feel 1 all-purpose name instead of specific-purpose is the best… but that's me.