Setup Slack app. If you have already setup, So skip it

Setup multiple Slack teams for a Confluence Space

Navigate to Space tool in given Space

Click on Slack tab

Click on "Connect" button

Choose your wish slack team at right top corner . My sample is "Slack Connector"

Click on "Authorize" button at bottom then you able connect "Slack connector" team to current Confluence Space

Click on "Connect" button , then choose other Slack team at top right dropdown

Now you are able to connect multiple Slack teams in to your current workspace.