Tomshardware reviewed a little switch that flipped the cable-select or master-slave pins on a pair of harddrives, it was recent so should be easy to find.
Or buy Partition Magic and set up multiple C: partitions on a single hard drive, then use Boot Magic to pick the OS you want each time you reboot (works great). Note that you need PM 5 or 6 for drives over 32 GB, but PM has supported linux partitions since at least version 4.