Sounds like a regular overflow box, like my DIY one here:
The side with the rolled up gutter guard is the side with the gate valve at the other end. The PVC sticking up in the other drain is the emergency.

The side with the rolled up gutter guard is the side with the gate valve at the other end. The PVC sticking up in the other drain is the emergency.
<a href=showthread.php?s=&postid=7211090#post7211090 target=_blank>Originally posted</a> by dandy7200
Could you explain how this would work in the meantime?