If I understand the planned design, the spa is under water as part of the pool level?

While it is a nice, sleek look, there is only one way to truly separate the spa from the pool when in the spa mode using this design.

Builders have installed a collection tank that allows the water level to drop down below the rim of the spa, so it can run and heat independently when set is SPA. Otherwise, your just heating the water from the pool and the spa together, and it won't reach temperature.

The collection tank must be sized accordingly to handle the volume of the water needed to drop down to the spa level. It's a tricky set up and a lot of calculations are required. There is additional plumbing and pumps to make this happen. Plus and additional structure of gunite must shot, or a fiberglass tank must be used.

Consider a perimeter overflow spa that is set 2"-3" above the pool water. The top is constantly wet and provides a nice look. And, you can heat the spa independently without the worries of interference of the pool water.

Exactly what I was looking for, thanks!