There are documents on planning the installation which should help you - here is the one for Solaris 8

Too many unknowns -

Is the OS and applications using these two drives together?
Will you be using Disk Management Software (SDS, Veritas...)?
What type of application?
How much space does the application require?
Are user home directories needed? How much space required?


Standards for the company I work for include
Separate disks for OS and applications
Mirror OS disks - Application owner needs to let us know what to do with their's (we recommend minimum mirroring)
Application DBA works with us to guesstimate the disk size needed for present, 6 months/one year/two years down the road.
Home directories for users not on same server - nfs mounts so they can't kill any of production (althought some still manage to do this).