Just an added comment here.

My current desktop (a DELL) came with Window 8 (later upgraded to 8.1). I installed a second hard drive for opensuse. It is a UEFI box. So this is similar to the situation that is being asked about.

I created an EFI partition (sometimes called an ESP) on the second hard drive. There was already an EFI partition on the first drive. I create a 500M partition for EFI near the beginning of the second drive.

During install, I set that 500M partition to be mounted at "/boot/efi", and to be formatted FAT. The installer took care of all of the other details for making it an EFI partition.

It turned out that this was a good setup. The UEFI implementation in this computer is broken, and using a second EFI partition works around that breakage.

I have a second computer with two drives. I also decided to put an EFI partition on each drive. However, that second system is working well with on the EFI partition on the first drive being used. I'm using the EFI partition on the second drive to backup that on the first drive. But it is booting entirely from the first drive. That's how it is supposed to work.

My advice to the OP: Create a second EFI partition on the second drive. However, use the EFI partition on the first drive for your opensuse install. The EFI partition on the second drive is just a spare in case you later decide to use it. By allocating the space for that up front, you make it easier to switch to that at a later time.

As far as I know, ASUS systems have a reasonably good UEFI implementation. But note that I have no personal experience with them.