QB Issue Resolution:

You really cannot apply a customer’s credit memo to another customer. ¬†You would have to create a second credit memo to the other customer’s name and put notes as to where it came from.

Resolution for Issue 'apply payment from another customer’s credit memo' available: Yes (Solved).
Source: Intuit Community forum.

apply payment from another customer’s credit memo: this 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 issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

Support for Credit Memo
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Error Recovery for Credit Memo [Canada]
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