You can link customer accounts by following these steps:
Find and open one of the customer accounts that you are linking.
Click Link customer account in the Linked customer accounts section.
Find and choose the other customer account that you want to link.
After you have linked the accounts together, the contact details will be synchronized automatically to both accounts when updated.
However, the orders will show only under the account that they are attached to. If you need to transfer orders from one account to another, please see the article Transferring orders between accounts.