So you want to cold boot from one partition, reboot, and have the machine boot from a different partition?
That won't be easy, as there isn't really a way to change settings in that early boot menu. (it's generated by hardware, really -- essentially, the only thing the Amiga has done by that time is to have completed its self-test and read the configuration from RDB.)
I think you could probably accomplish the same thing by booting with only one partition and using some creative startup-sequence scripting. Basically, have your first boot set up a RAD or something, and then do an IF EXISTS at the startup-sequence to see if RAD exists, and if so, assign SYS:, LIBS:, S:, etc, to the new drive, and then start executing startup-sequence from that drive.
Technically, you'll still boot from the same first partition twice, but on the 2nd boot, it'll hand over control to the 2nd partition. Probably the best you're going to get without doing some REALLY ugly and possibly dangerous tricks (like having a script that alters RDB!)
