So your looking to get to the boot screen between both your hard drives while there on seperate hard drives?
Don't get me wrong, I'am pretty sure that you can 'select' either ubuntu or xp in the boot up process, but, I might be wrong. My experiance with dual booting is the bootscreen it'self and both operating systems being on the same hard drive. You can split things accordingly between them(but this would be to much work, you would realistically create a partition for linux on your second drive and run windows and ubuntu on one drive) and access them differently, hm, I don't know, usually having the different OS on seperate disk can run into problems because of your primary hard drive booting first.
setting up a dual boot on separate hard drives - TechSpot OpenBoards
I looked around on the details of what you want to do, you basically have to edit your bootini file to give you some time to choose how/which os you want to start from, I would suggest looking into your boot ini, probably pasting it so some of us can take a look.