How can I create a bootable usb flashdrive that works with UEFI and BIOS computers for installing Windows 8.1?

I manage a lab with a lot of computers: almost half of them are BIOS-MBR computers and the others are UEFI-GPT. All of them run Windows 8.1. As you know, you can't use for booting a mbr flashdrive from a uefi computer or a gpt flashdrive from an old style bios computer. In case of uefi machine you can force it for working as a bios-mbr computer but i dont' want to do that. For obviously management reasons I'd like to create a single flashdrive device for installing Windows 8.1 when I need to do it that can be used with old (BIOS-MBR) and new (UEFI-GPT) machines. At the moment I use two different flashdrive devices: one gpt style and one mbr style. I noticed that the usb bootable media created by Acronis True Image 2014 is good for bios and uefi computers. How can I create a single bootable usb device like this? Thanks for helping. Bye