QB Issue Resolution:

You can create and use as many company files as you want when using the QB Desktop version.  There is no charge per company file.

Resolution for Issue 'Two different business, two different owners. Do I need two separate subscriptions (desktop version) for these business’' available: Yes (Solved).
Source: Intuit Community forum.

Two different business, two different owners. Do I need two separate subscriptions (desktop version) for these business’: 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 company files
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Error Recovery for company files [Canada]
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Additional Support Options: