You could do try this by just placing the Grub 2 boot loader into the MBR of your internal hard drive and the rest on the SD. I would think though you would want to also separate out the /boot folder and place it on the internal hard drive as well so you could boot something other than the SD card later. This will require a custom partition setup and a modification of the booting section and making sure not to put into jeopardy your personnel data while trying to make this work. So make a data backup before you do anything else.
So, you must select a custom partition, not based on anything openSUSE comes up with. You need free space somewhere or you are downsizing existing partitions or you are deleting them. You can use an existing SWAP and /home partition if you want to. You should then create a /boot partition on the internal hard drive (/dev/sda) and it likely should be a primary partition (sda1, sda2, sda3 or sda4) of 2 to 4 GB’s in size. That means you add a partition, make it EXT4, a primary partition and mount it as /boot. You will want to think about where to place your /home folder and the SWAP partition. Next, for root, make a EXT4 partition on the SD drive, which must be installed before you boot the CD or DVD and assign it to root /. Create a EXT4 primary partition there and I would hope you got at least a 16 GB size SD Card. You normally need at least three partitions total between all drives, the Root /, SWAP and /home and you are adding in a forth for /boot. Before you select install In the booting section, you can select a custom boot location and make sure it just says /dev/sda with no partition number. Before you elect to continue the install, it should say in the boot section to install Grub into the MBR. You know you can go through the paces but just abort before you allow the install to continue and write down any new questions you have and don’t allow the install to continue. I have a write up on partitioning, but not to do what you ask, but still helpful none-the-less.
Of course you can make a seperate partition for /boot when you want. Linux does not care about the real hardware (revolving disk, static memory) beneath it. And Linux also does not care about where the partitions are (on different devices). So the mixing is up to you. In your case, a /boot aprtiition on a device the BIOS can boot from and GRUB in it’s MBR looks fine to me. You can tell the installer what you want (expert mode or such wording in it’s partition part).
I wouldn’t spread the system across SD and internal HDD, rather first make sure the laptops won’t boot from other devices.
If they can, install to the SD card. That way you can install the system supposed to run from SD on the SD, leaving the rest of the machine untouched and not needing the SD card for booting.