Can I assign a Salesforce user to both Genesys Cloud for Salesforce and Genesys Cloud for Salesforce Service Cloud Voice integrations?

No, you can assign a Salesforce user either only to Genesys Cloud for Salesforce or Genesys Cloud for Salesforce Service Cloud Voice.

You can have both the setups Genesys Cloud for Salesforce and Genesys Cloud for Salesforce Service Cloud Voice 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 Salesforce documentation.