I have a dual booting system set up in which there are two operating systems on two different drives.

1. Disk 1 (HDD) - Windows XP x64 Professional (Uses IDE)
2. Disk 2 (SSD) - Windows 8 x64 Professional (Uses AHCI)

Now I can use Windows 8 if I change my BIOS to AHCI and not Windows XP. While I can use Windows XP if I change to IDE. How can I set up either of the OS so I can use whichever I want without having to change BIOS Settings again and again?

Since one works in one mode and the other works in the second mode I have a feeling you switched modes when you installed the new OS/drive. If that is the case then you will likely also have to edit the boot loader to add the new OS. (I am assuming you aren't seeing the option to select an OS when you boot?) It is possible that once the boot loader knows to give you an option that it will work just fine.

More likely XP will blue-screen because it doesn't know how to handle AHCI mode.

It is supposedly possible to switch from IDE to AHCI in XP, see a discussion at http://www.neowin.net/news/neowin-guide-how-to-change-from-ide-to-ahci-without-reinstalling-windows if you want to give it a shot. I would recommend backing everything up first, of course.

Good luck!