If you don't want to delete either, maybe it's worth the $0-$2/month to keep them accessible?

But if not, maybe you should call or web chat with customer service and see if they can "deactivate" one. In v1.0, when each extra character was around $1 each per month, you could see characters under an "options list" of your service account, and choose to cancel or activate them individually, without deleting them. Of course the game bills differently now, and that may no longer be possible.

Even with the deactivate route, the character would be completely inaccessible, unless you could transfer to another server and activate it. But that might cost $25, more than a year of keeping both characters active, and even then the two characters would be completely isolated from each other.