When your system runs low on physical memory (RAM), it will use a page file to extend available memory. If you have two hard drives, you can use two paging files – one on each drive – to speed up disk access of your page file.
This optimizes page file performance because your PC’s hard drive controller is able to read and write simultaneously from more than one hard drive. What’s more, based on its algorithm, Vista will use the paging file from the partition that is accessed less frequently. By default, this will be the second paging file on the separate hard drive. As such, using two paging files will speed up the paging process and boost system performance.