Did you put them on separate hdd's? If so then go into bios at each start up and choose the hdd containing the os you want as first boot device, (or at least everytime you want to switch from one to the other).
im having same problem, I have win 8.1 32Bit installed on one ssd and win 8.1 64bit on another and I cannot get the screen to choose which one to boot from. I use 32bit for older games that wont be compatable with 64bit. Before I reinstalled win to one of my ssd I had the option screen to choose from now its gone