Hi,
You cannot change your single login to dual immediately:
1. You have to wait when your subscription is finished, only then you will be able to change single login to dual one (in the billing).
2. If you change your login to dual, you will be charged 50% more of your single subscription.
3. How is it possible that your client has dual-login? Did you know about that? If you are reseller, you had to register your client and pay for his subscription by yourself... From your post, looks like you don't know what is reseller, your clients and how to...
4. Are you really reseller or you just want to be reseller whithout knowing how to do that?