Does the AmigaOne have the "early boot menu" and can it be used to boot different OS's (Or even varients of the same OS eg "flavours of Linux")
The BIOS will allow you to choose the disk to boot from.
Additionally, the original early startup menu will still be there. This can be used to boot from different disks as well...
The bootting process on the A1 is actually a two-stage approach: The first stage is carried out by the bios and loads the OS4 kernel (which used to be in ROM). The second stage is the "normal" boot that is carried out be Amiga's DOS.
The first stage is always carried out when you turn on the machine. On subsequent boots, you can skip it (the kernel will stay in memory, then), or choose to "cold boot".
Dual booting can be carried out by both, although it's probably better to cold boot and use the BIOS to boot into non-OS4 systems (like, uh, Linux, and... Linux

And before anyone asks, no, cold rebooting does not mean you have to turn your machine off and on again...