Can I connect multiple Slack instances into a single Zendesk instance?
No. It is not possible to integrate two Slack accounts into one Zendesk instance. The slack integration. can only handle one account.
If your Slack Approve apps permission setting is off, you can potentially break the integration when another instance is installed. We recommend turning on the Approve apps setting in Slack, or advising your Slack users to not reinstall the integration.
For more information read this article: Understanding, installing, and configuring the Slack for Zendesk Support integration.