Yes, that is possible. All partitions other than the system partitions (typically /, /usr, /var and /opt) can be shared by the two OSes. All partitions, including the system partitions, can be mounted and accessed by either OS.
The easiest way to set this up is to do separate suninstalls on two different disks. Then just choose the appropriate disk at boot time with the PROM's "boot" command.
Setting up both OSes on one disk is a little harder, but not much. You need to partition the disk to allow for both OSes. Almost any partition layout is possible, but one common setup might be:
a: / for Solaris 2 b: swap (shared) c: The usual (whole disk) d: / for Solaris 1 e: /usr for Solaris 1 g: /usr for Solaris 2
Again, it's most reliable to use suninstall to do the installations. If for some reason you choose not to use suninstall, make sure you run installboot for both bootable partitions.
With this setup, you choose between the two OSes in the PROM's "boot" command as follows:
To boot Solaris 2: boot To boot Solaris 1: boot disk:d
NOTE: In boot PROM versions <= 2.5, the "disk:d" syntax is not supported, and the PROM cannot boot from root partitions that begin or end beyond 1GB.