1) What machine do you have, as it will affect the advice
2) Im assuming, since you have 2HDs, you have a PowerMac (G4 or G5)
You cant use both to speed up Logic 8 (And it wont speed it up really either) - what you need is to max out the RAM (if you havent already done so), as that is the thing that speeds Logic up the most - If 1 HD is faster (more rpm) than the other, the only thing you could do is move your Samples, project files and loops across to the speedier drive - thats about all you can do I think.