Dual boot for Win8.1&Ubuntu using GPT with MBR legacy mode

I'm pretty sure this is the wrong part of the forums but...  


I need some help with my setup, I have 2 computers and 1 SSD. One computer can boot of the GPT SSD but the other cant(Its an old Dell Optiplex 330 with everything upgraded but the MOBO). I want to be able to have the SSD be able to boot Windows 8.1 and Ubuntu on both computers. I have no idea where to start or if this is even possible. I at least need to find out if I can boot Windows 8.1 on both computers. This stuff is just out side my comfort zone so try to make it as simple as possible if you can.


Thanks for all the help,