At the moment in integration manager for Slack there is only option to use obsolete webhook method which allows sending notification to single Slack channel configured for this webhook. There is (not that) new API method https://api.slack.com/methods/chat.postMessage which allows sending notification to multiple channels. This is extremely useful if you for example want to differentiate alerts based on its kind or severity. There is also option to use Webhook method in MongoDB but it doesn't support Slack. So please either add support for new API method or make Webhook method supporting Slack so at least two Slack channels will be available to use.