You can save them as PDFs and attach them both to one invoice.
Resolution for 'attach issue: 'Can two or three invoices be sent in one email?' available: Yes.
The attach 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 attach issue yourself by looking for a resolution described in the article. If it is a complex attach issue or you are unable to solve the issue, you may contact us by clicking here or by using other attach support options.
For more information about resolving issues related to attach in Quickbooks/QBO or to get support for the issue 'Can two or three invoices be sent in one email?', click the link below: