Apparently QBO is deficient in this important area and you must receive everything on the PO at one time.  A workaround might be to delete the backorder from the PO and re-enter a new PO for the backorder.

Resolution for 'AND create issue: 'Can I keep a purchase order open for unreceived items AND create a bill for items received?' available: Yes.

The AND create 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 AND create issue yourself by looking for a resolution described in the article. If it is a complex AND create issue or you are unable to solve the issue, you may contact us by clicking here or by using other AND create support options.

For more information about resolving issues related to AND create in Quickbooks/QBO or to get support for the issue 'Can I keep a purchase order open for unreceived items AND create a bill for items received?', click the link below: