Is a dual boot with SSD and SD possible?

Hi,
I am fairly new to this and have a question.

Is a dual boot with SSD and SD possible? A DietPi is installed and set up on the SSD. Now I would like to run an SD with DietPi on the RPi4 at the same time as the SSD and both should be booted at every reboot.

Thanks in advance for your answers and help!

Both cannot be booted. One of them can be booted at every reboot.
Maybe you want to explain what are you trying to achieve?

At least on RPi, SD card should be first device within the boot order. Usually SD card would need to be removed to have the system booting from SSD. Or you change boot order.

The EEPROM bootloader on RPi 4 can be configured to change the boot order: Not from USB even when an SD card is attached. But AFAIK there is no way to boot two OSes concurrently, I wouldn’t even know how to do that on an x86 system. Of course a VM or container wound be possible, i.e. booting a second system via systemd-nspawn as container.