You are not allowed to create another character on a merged world if at least one of your characters has been moved to a merged world.

However, if you had a character on multiple game worlds that were merged into a new world, you are still allowed to play these and you won't have to fear any sanctions due to multi-accounting. Of course, this only applies as long as you behave fairly and do not use your characters to gain advantages for one or more of your characters.