at first we have two system disks, but what you say is not correct.
The executables are different. that's right. But the filesystem is the same.
if you are able to create two VMS$COMMON (e.g. VAX$COMMON, AXP$COMMON) and
you are able to to setup the system logicals (SYS$COMMON, SYS$ROOT ...)
and the directories (through SET FILE/ENTER=SYS$SYSDEVICE:[SYSA]SYSCOMMON.DIR
AXP$COMMON) correct and at the right time, it should be possible to have only
one systemdisk. But AFAIK you have to change STARTUP.COM and that should you
never do that!!!!!!
Also, there could be some side efects which make it impossible to do so.
Regards R. Wingert