You manually enter the data you want on the invoice you want to send & then delete the other invoice. There is no way to merge the invoices.

Resolution for '2 invoices issue: 'How do I print 2 invoices onto 1 invoice?' available: Yes.

The 2 invoices issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the 2 invoices issue yourself by looking for a resolution described in the article. If it is a complex 2 invoices issue or you are unable to solve the issue, you may contact us by clicking here or by using other 2 invoices support options.

For more information about resolving issues related to 2 invoices in Quickbooks/QBO or to get support for the issue 'How do I print 2 invoices onto 1 invoice?', click the link below: