While you're able to define multiple web managers, we recommend against having more than one enabled at the same time. The manager resource was designed to operate independently, and having two of them manage the same dynos (i.e. web) will result in undesirable scaling behavior.
We may add the ability to autoscale web dynos based on multiple metrics on a single web manager in the future (i.e. response time + load). We currently have no plans to do so.