You obviously know Linux and RAID so I won't go into huge detail. Just post back any questions.
1. With Solaris running enter "format" and write down what volumes you can see (eg. c0t0d0, etc). These are disks already configured. Shutdown the system.
2. I don't think it matters which slots you put the new drives into.
Configure a new 2-disk RAID0
Configuring Hardware RAID - SPARC T4-1 Server HTML Document Collection
3. Boot up Solaris again. Run the format command and you will have a new disk volume (as compared to (1) above). This is the one to format, newfs, and mount. Don't touch the originals as you will screw your existing O/S.
Hope that helps.