It’s possible to install same version different years, but unless you need to share the Ent. 8.0 version with someone else who has only that version you could just open that file with Ent. 18 (although you might have to “step up” the upgrade)  – ??

Resolution for '18 Desktop Ent issue: 'Install two versions of QB(8.0 Ent &18 Desktop Ent) in the same machine?' available: Yes.

The 18 Desktop Ent 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 18 Desktop Ent issue yourself by looking for a resolution described in the article. If it is a complex 18 Desktop Ent issue or you are unable to solve the issue, you may contact us by clicking here or by using other 18 Desktop Ent support options.

For more information about resolving issues related to 18 Desktop Ent in Quickbooks/QBO or to get support for the issue 'Install two versions of QB(8.0 Ent &18 Desktop Ent) in the same machine?', click the link below: