It looks like you have 3 lines going into your sump. 2 inlet - 1 refugium. You are going to need a gate valve to control the supply to the refuge from the return pump. Instead of a "T" off of the return with a gate valve to control flow, you would "Y" off 1 of the 2 inlet/drain lines along with a gate valve to control flow to the fuge. Your system will still maintain the same drain capacity. All it would do is lessen the volume of water going into the the inlet/live rock chamber.
Use gravity when you can instead of the pressure of a return pump to supply your fuge. Keep the return as simple as possible A to B try and eliminate all catastrophic leak potentials.