You need to add an "overflow" pipe in the spa that would basically replace the current spillover.. So, instead of spa water going over the spillover, it would go into this new overflow pipe and into pool... In theory it would be hidden and not seen.
Spillovers work because no matter how much water you put into the spa, all the excess spillover into the pool..
You also need to keep the same basic Intake and Return valve operation so that you can still switch between the pool mode and spa mode.