When merging customers all the information from the "losing" name will disappear. Thus, the option to unmerge them is unavailable.
If you make a copy before merging, you can enter the customer's contact information manually or import them into your QuickBooks Online account.
On the left panel, select Sales.
Select the Customers tab.
From the New customer drop-down, select Import customers.
Select Browse and find and select the Excel file.
Click Open, then select Next to continue.
Map your data then select Next.
Adjust your data if necessary then select Import.
The following article contains more information about importing contacts from Outlook, Excel, or Gmail, as well as other kinds of imports like bank transactions and products and services: Import customer or vendor.
Drop me a reply below if you have any other questions while working with QuickBooks. I'll be happy to help you some more.