If your current system has two drives now and you still have your optical drive working you maybe able to convert it to a third drive. Otherwise your limited to the number of SATA ports you have.
As to running with two SSD's I wouldn't recommend that, while there is nothing stoping you in doing that you technically. You're not gaining enough performance and you are loosing storage as the HD is still larger than the SSD you're replacing it with. I would likely up the SSD size and maybe upgrade the HD to a faster & larger drive.
The best layout currently is to have your SSD the boot disk directly on a SATA port and then having your HD on either on the second SATA port or via an adapter (PATA to SATA) with the SATA HD mounted in the optical drives location. But, until you tell us which model you have this answer could change.
And yes, Mountain Lion would be the best OS to use with a SSD setup.