to have 2 company files open at the same time, you need Accountant or Enterprise:

https://community.intuit.com/articles/1577589-open-and-use-a-secondary-company-file

Another solution is, when you upgrade, leave one of the files in the older, e.g., 2017 version and upgrade the other file to the new/2018 version.  You can have 2017 and 2018 […]

Resolution for '2018 update issue: 'with the 2018 update multi monitor, can I open 2 companies at the same time?' available: Yes.

The 2018 update 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 2018 update issue yourself by looking for a resolution described in the article. If it is a complex 2018 update issue or you are unable to solve the issue, you may contact us by clicking here or by using other 2018 update support options.

For more information about resolving issues related to 2018 update in Quickbooks/QBO or to get support for the issue 'with the 2018 update multi monitor, can I open 2 companies at the same time?', click the link below: