You can accomplish this by a gravity return to the pool. It just has to be set up so that the spa return doesn't put more in than the gavity return can take out. You don't need an infloor system with a sunshelf. You will have to brush the shelf down ocasionally to get the settled dirt off and into the path of your cleaner.

We will have a spa that is raised 18" (7ft diameter) and it will all run on the same equipment. Ours will have a spillover but it doesn't matter. We will have the option to adjust the spa so that we can shut down the pool in the winter and use the spa and it won't spillover onto the cover. Can I ask...why does your husband not want the spillover? I wanted a water feature and not a spa and my husband had to have the spa. We are both getting what we wanted b/c the spillover is like a "free" water feature for me.

Of course you can... You can eleviate the spillover notch all together ... And the spa will function like a raised pool... When you switch to spa mode it will direct the flow from the pool return to the spa jets... I do this quite a bit with detached spas...

The problem, is that when the system cycles between pool and spa mode, gravity will cause the spa to drain. Normally its not an issue if there is spillover, or overflow pipe that goes back to the pool to replenish the spa level.

If you want 2 isolated vessels, you are better off going with 2 separate equipment sets.

Thanks everyone! I see that there are things to take into consideration with this one. I'm glad I asked. Megankheaps, my husband doesn't like the noise of water features.

Racket,

If you use a front load skimmer to return the water back to the pool then the spa would oly drain down to the throat just like a spillway.

Even easier you could use and overflow piped back to the pool.

Take a look at my pool.......
http://ths.gardenweb.com/forums/load/pools/msg0517392715194.html

We didn't use a spill over at all. There are 2 drain pipes discretely placed at the water line of our spa (which is 30" higher than our pool, and 8 feet away)
If the spa water gets too high, it will spill through these pipes into the pool below the water line of the pool. No noise.
In circulation mode, water will be sucked through the pool skimmers, through the main pump, and back to both the pool and spa. Everything gets filtered and chlorinated by one variable speed system.
Also, it was very easy to do. 2 pvc pipes, and a check valve. No additional cost at all from the builder.