Unfortunately, unmerging two customers cannot be done. Here are some important things to consider in restoring both accounts as best you can.

If the missing customer had a credit card on file, its now been added into the remaining customer's account. You can remove this credit card info from the customer's profile using the Actions dropdown in the top right.

If you're unsure which card belongs to which customer, it might be safer to delete both, then ask each customer to re-add their card info. When asking this, you can share this article with them to help them self serve.

Did the missing customer have packs /credits / subscriptions that were merged into the other customer's?

Lets remove these first. Navigate to the existing customers account and delete these as needed.

You can do this from Customer List > New Customer.

After the missing customer's been added back, we'll now add their credits / subscriptions / packs back onto their account via 'New sale'.

These can be added back for free as the customer has already paid and we don't want to double charge them.

(Note: Since subscriptions typically charge once a month or fortnight, we'll first ring the subscription up as free, but then edit the subscription's renewal schedule / price in the next step -- to make sure it charges the right amount on the right dates going forward.)

Before confirming the free buy, consider unchecking this box before confirming the purchase so as not to notify the customer.

With subscriptions, after selling this month for free you'll want to edit the subscription's renewal price / date. Click the dropdown menu to the right of the subscription > edit.

Change the renewal price here and edit the 'Renews on' date (top right) as needed.

Don't forget to scroll to the bottom to save your changes.