can you Dual boot with same o/s? - Yes
1, Can he do it? - Yes
2, Does Windows have the software? - Not that i know of. (98 at least)
Install 98 to the other drive and when he wants to boot into the "other" 98 have him swap the boot device to the other drive. Do this in the BIOS...
You could use a bootloader to select when the computer starts but i have not used one in this case.
click here maybe?