Dual Booting, also sometimes called Multiboot, is a process by which you can install multiple operating systems on your device, and select between them when booting it up.
postmarketOS doesn't currently support dual booting through recovery, yet there are a few ways this can be achieved:
- On the Nokia N900, u-boot provides a mechanism to select the OS. See Nokia N900#Installation
- On fastboot devices that have an SD card, it is possible to install postmarketOS on the SD card, and boot through
pmbootstrap flasher boot. This has the advantage of not modifying anything on the phone itself, but a USB cable and a host device (like a computer) are needed to boot the device with this procedure.
- On (?some?) fastboot devices, the recovery partition holds nothing but a second fastboot boot image. With the postmarketOS boot image flashed to the recovery partition and the root image flashed to either an internal partition or an sdcard, you should be able to dualboot (this works on xiaomi-santoni and samsung-degaswifi)
- A slight variation of the above works on harpia: flash lk2nd+pmos boot image to boot partition and android boot image to recovery. Flash root image on sdcard. Run `fastboot oem fb_mode_set` to force the bootloader menu screen on every power up/restart.
- efidroid might be used together with postmarketOS, but has not been tested
- Probably multiboot can be implemented with any second stage bootloader, capable of running linux (for example lk2nd)