Depends on workload of course, but for most VMs they read a lot more than they write so an SSD can handle quite a few before you notice any IO-related latency (3-4 small web-apps, 1-2 medium, 1 database VM).
The OP also put his two drives into RAID 1 so they won't get any benefit of having two drives.
I use mdadm's RAID 10 on 2 drives for redundancy and speed.
"In mdadm, the RAID10 level creates a single complex software RAID that combines features of both RAID 0 (striping) and RAID 1 (mirroring). Multiple copies of all data blocks are arranged on multiple drives following a striping discipline." http://doc.opensuse.org/products/draft/SLES/SLES-storage_sd_...
You do have more chances for the spindles to be near your data (especially with >2 drives in the array), which can slightly reduce average latency, but that isn't as large an effect.