Hi @SC_10187722.
This is not a supported configuration. Hosting 2 instances on the same system is prone to issues.
It's recommended that you set up a clone of your 8.5.1 instance on a separate VM. Then use your clone to test the upgrade procedure and regression test all your apps. Custom extensions and mashups may require update under the 9.4.x version.
Depending on the process you use for setting up your clone, you'll need to be careful of schedulers and timers, as well as integrations with external systems, to ensure your production environment is not compromised.
Regards.
--Sharon