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. You are allowed to play such characters in the same guild.