I think that the water flowing from the spa to the pool is a nice water feature that you essentially get "for free" when they are attached.

So, a detached spa is essentially a second (albeit smaller) pool? Maintaining chemical balance for two pools instead of one, extra plumbing, and perhaps extra pumps?

If this is the case, it is probably easier and less expensive to get a stand-alone pre-fab spa - and then you get the luxury seating and padding, extra jets, etc. Then you can also empty it during the off-season.

Anyway, I've never seen a detached in-ground spa. I have a spillover spa and I love it. We put some extra seating on either side of it, so folks who don't want the heat can sit in the main pool and still talk to folks inside the spa.

Here is a link that might be useful: diagram of our pool spa and nearby seating

Depending on what you want stick with it. A separate spa raised within 10ft I would plumb in 2 2 1/2" overflow drains that are visible at your tile line down side. Both pool and spa. This is very critical that there set @ gunite/shotcrete. This so you have a single system. Cheers I have many ideas, a raised spa attached to a pool with a spillway looks great but if you have the room go separate. If you have trees consider a skimmer in the spa its just one extra line. Of course expect some extra costs in check valves, automating the Eq. Goodluck