Not 100% what you're trying to say here, but I have an idea
You won't be running DX9 and DX10 "at the same time." The way DirectX currently is, is that it has full hardware support for previous version. Full backwards compatibility. Which means that if you get DirectX 9, even though it "overwrites" the older version, you'll still have full functionality with DirectX 8, 7, 5, 3.3, etc.
Doing this is nice, as it provides full hardware support, so if you play a game that uses DirectX 8, you'll still see full hardware support.
With DirectX 10, they have decided to start with a clean slate with no backwards compatibility. This means that if you play a DirectX 9 game, it's going to have to do software emulation, which is typically slower (though how much depends entirely on the implementation). At first this seems to suck, because you may get significantly worse performance. However, it also reduces a lot of code bloat, which can make the DirectX 10 applications run faster than otherwise (if DirectX 9 had no backwards compatibility, it's entirely possible that DirectX 9 games would run faster that it currently is). If the performance of legacy DirectX is still good (which will depend largely on the CPUs, and probably the graphics card, since it'll be emulated), in the end this could very well be a good decision.