It shouldn’t matter much whether the data is on separate drives or the same drive. It matters a lot with mechanical drives, because the heads have to seek for data, so it’s better to have one drive seeking for one bunch of data and another drive seeking another, rather than one drive doing all the work. There is no seek time on SSD’s, just the time it takes to read from memory. Although theoretically there might be a slight boost to use more than one drive for different purposes.
But SSD’s are expensive and the largest single drive capacity is limited to 750 or 980 (but the 980’s are really just two 480’s spanned together). The smaller ones are more economical (I mean less money per GB) than the larger ones, but there is no way I could afford to have all of my sound libraries (totaling around 4 TB) and project archives on SSD’s. And I would prefer to keep my project archives on an external mirror RAID for safety reasons.
Having said that, you will likely see a performance boost if you RAID two SSD’s in a RAID 0 (striped) configuration for your boot drive, especially if both are 6G SATA.