Unfortunately, this is not currently possible in Tradify.
As a work-around, if you have two or more customers that are the same and both have invoices, quotes, purchase orders, bills or jobs assigned to them, then you can simply change the customer record on each of the current jobs and/or documents to the customer record that you're wanting to keep.
In order to do this, your documents will need to be in a draft state.
Once you have changed the customer record on the relevant jobs and documents, you can go ahead with archiving or deleting the unwanted customer recorded. To do this, open the customer and select Options > Archive Customer or Delete Customer.