Can I create two shelves into one big raid group in order to take advantage of all the spindles, and then create two volumes on that raid group?
No. A raid group belongs to one and only one volume. Spares are the only disk that can be shared between multiple volumes.
However, give some thought to whether you need separate volumes. Do you have different snap* (snapshot, snapmirror, snaprestore) requirements for each one? In most situations, a single volume w/ separate qtree's is sufficient. (though I still prefer root volumes of only 2 disks).
..kg..