Normally speaking, any dedicated return line to the spa for the purposes of a spillover effect will be located on the POOL side of the return plumbing. So, in order to take water from the spa, heat it and return it to spa for simple recirculation would require some tricky plumbing and at least two valve actuators.
A better approach might be to have two dedicated returns to the spa (not jets) where one comes from the POOL side of the plumbing, and the other comes from the SPA side of the return plumbing. Then you can simply put a 3-way splitter valve between the jets and the single return allowing you to choose between returning heated spa water through the jets (causing lots of aeration) or to the wall return (recirculation heating). The other wall return would come from the POOL side of the return plumbing so that a spillover waterfall is created when running in POOL mode.
A picture of your equipment pad might help the conversation a bit.