I had a customer with a similar problem, but different. They had two separate systems, but wanted a constant water exchange. There was no way to connect the two with and equilizer or spill over without a major concrete excavation and repair. I did the water exchange at the equipment pad utilizing a diverter valve to control input into the pool body. See the attached picture. This can be very touchy and either overfill the spa or suck it dry. To compensate I inserted a solenoid valve on the return side controlled by a Levelor. The diverter valve just needs to be set at a lower flow rate less than the flow rate of the solenoid valve. The solenoid valves come in sizes from 3/4" to 2" and can be purchased at any irrigation supply company. I was hesitant to try a water exchange at the pad, but it has worked very well. I have since done the same thing at another pool that had too small of an equalizer line. This particular spa was mounted higher in the landscape than the pool, but no spill over. It "spilled over" through an equalizer line. They regularly have lots of bathers and the one 1.5" equalizer line didn't turn over water fast enough. Under large bather load, the tub would get pretty scummy and require a manual intervention to drain and refill. We added two more equalizer lines and use a Levelor to control the water inflow. I put in a switch to manually override the water sensor to turn off water "spill over" when desirable. This allows the owners to heat up the spa without loosing heated water into the pool. They can take a spa and when finished turn the water turnover back on with the flip of a switch.