Technically there is room for one 15mm drive as long as it is paired with a 9mm drive or smaller. You can see that per iFixit there is about 26mm of internal height inside. With that said, most likely the SSD will not be able to be mounted with the rubber gromets (my guess is that it won't line up) so the SSD will probably "float" on top of the hard drive. This is actually how I had mine in my 2011 for over a year with no issues (I didn't purchase the necessary gromets so I just slapped it in on top of the hard drive and left it).
Caveat: I'm not sure anyone has truly tried a 15mm + 7-9mm SSD, but physically there is room.
But will the drive seat into the grommets? If they don't align then it won't fit. (I assume you're talking about the bottom bay where the standard HD is since it is larger than the top bay.)