I have native eSATA port, ofc. I already had installed Windows 7 on that drive, so i am sure it is possible.
To be more specific, its GA-Z68X-UD4-B3 and WD MyBook.
So... I am going to turn off AHCI for eSATA, try to repair external XP's partition with install disk (or some other software) and then it should be normally bootable. Then swtiching between OSes using boot order in BIOS.
I know there is option to install XP first then W8 and the newer OS will recognize the older one then let you choose between them (without using BIOS). But thats not really possible since i am not going to reinstall neither one of those OSes.