Pspeech, I may be misunderstanding your question. Are you asking how do I keep water from the pool getting in the spa?
If so, my pool and spa have different water levels. I usually keep my pool water level around 3" below coping (middle of the waterline tile). The negative edge on my spa is exatly 2" below the coping, so it's 1" higher than the pool water level. I'm sure if all 3 of our kids and my wife cannonballed into the pool some would get in the spa but not enough to cause problems heating the spa. In cases of rain, my pool overflows would capture the water before the pool overfilled into the spa.
Here are some pics of the spa/pool, ignore the mess - it's still a bit of a construction site.
From the deep end opposite the spa, you can make out the spa height difference. From the spa side looking towards the deep end, you cannot make out any elevation difference. This was done intentionally with the negative edge so it all looks like one body of water, even though there is a 1" height difference.
The funny thing is, every morning I have a frog sitting on the high edge of my spa looking out into the pool. He sits right under the coping on the spa edge. Everyday my kids take him away and put him in the wooded area behind our house. Today he was there with a friend. I'm hoping I don't wake up to 3 frogs on my spa edge tomorrow. We'll see....
