Can I assign a Salesforce user to both Genesys Cloud for Salesforce integration and CX Cloud from Genesys and Salesforce integration?

No, you can assign a Salesforce user either to Genesys Cloud for Salesforce or CX Cloud from Genesys and Salesforce.

You can have both the setups running together in your environment, but you cannot assign a Salesforce user to both the setups. For other limits and limitations for Service Cloud Voice, you can refer to the Salesforce documentation.