Put 2 SSD's in imac without removing Optical drive

Its definitely possible. That iMac (same one I have) has three sata connectors on the logic board. But you have to remove the logic board to attach the connector, and its a bit of a pain to get back in place.

But from what I read, if you pull the hard drive and replace with a SSD, the SMC will get confused not seeing the hard drive anymore, and it will ramp up the system fans. You can override the fan speeds manually (I have to do it until I fix mine), but I would say its a suboptimal solution.

At the very least I'd suggest leaving the HDD and replacing the optical drive - I did that a while ago to make a fusion setup and am quite happy. I have an external USB bus-powered optical drives for the few times I need it.

Though after just having broke my iMac during a re-assembly, I'd strongly suggest you consider going with a thunderbolt solution instead - I have the Seagate sled, and it works fairly well. I think the desktop variant even supports two drives, so you could dump two SSD's in there, and never disassemble the mac.

To clarify, you actually need a HDD with compatible firmware for the iMac, or it'll spin the fans like crazy - so you can't even swap in a hybrid (though my experience with the first gen seagates was incredibly disapoointing). Makes it a bit of a pain even if you want to swap out the HDD for another.

As far as what you can put on what, OSX is pretty gracious when it comes to copying a drive over (whether using SuperDuper or Time Machine). I've moved between the original, a TB SSD, and my final HD+SSD fusion setup without any problems. Windows is another beast, though Winclone helps out somewhat.

Again, I'd say its better if you don't have to crack the case. I consider myself intermediate/expert (20+ yrs), and the 27" is the most difficult disassembly I've ever done -- much, much harder than the pre-retina macbooks (though perhaps the new retinas aren't very easy), or any normal PC desktop.