To start with here's the dual drive install gulde: Installing MacBook Pro 13" Unibody Mid 2012 Dual Hard Drive.
As to whether your SSD will work: The Optical drive SATA port in some in this series often can't run at the SATA III (6.0 Gb/s) speed. Here's a bit of a write up OWC has on the subject with their Data Doubler Review the Note at the bottom "MacBook Pro 13" models: Apple does not support the use of 6Gb/s drives in the optical bay. While we have observed a high rate of success using SATA 3.0 6Gb/s drives in Apple 13" bays where 6Gb/s link is present, some systems may not operate properly with this setup." While OWC recommends swapping drives around if you have a SATA II (3.0 Gb/s) drive. Apples stock drive in most of these systems is a SATA III drive.
So you could chance it with your SATA III SSD, you could use a fixed speed SATA II SSD, you could swap the drives around if your HD is a SATA II, or you could go with a SSHD swapping out your HD for a hybrid drive.
We don't have very many 13" MacBook Pros, mostly we use 15" models. So I can't tell you from our experience what you'll encounter here. We at one time used the dual drive setups with a SSD but we often encountered the HD crapping out when we moved them over to the optical SATA port more so than leaving them in the original placement. We then started using Seagate SSHD's and haven't looked back.
Hope that helps! Good Luck