You also need to factor in the location of actual picture files....although 'most' of the raw image data is pre-rendered and stored in the CameraRaw cache, I understand that not all of it is. So even when working in Develop you will (initially) be hitting BOTH the CameraRaw cache folder AND the image folder....that's certainly my experience when just scrolling through my images in Develop.
FWIW, and based totally on input/advice received from this forum, I have 4 internal drives....one for OS/Program Files, one for Catalog/Previews, one for Cache, one for Picture files.
Where you have, say, only two drives then you have to make more of a decision as Mark suggested, based on your own use of Lightroom. One thing to note, however, is that you can always try different combinations until you get the setup that works best for you....as you say you cannot separate Catalog from Previews, but you can MOVE them from one drive to another. Similarly you can change the location of the CameraRaw Cache, and of course you can also change location (carefully, i.e. preferably using LR) of the picture files.