I know this thread is old, but still got me usefull.
I have an old iMac 2011 still in hard work this days.
Manage to buy one of those dvd adapters for ssd, removed the old optical, and already had the second disk, another ssd installed glued behind the optical bay.
I used the original apple cable to the optical, because the bay do the connector convertion to ssd, and in the other side, i cut the mixed power data cable from it in half, and exchanged sata 0 e sata 1 connections (so to have my original HD with thermal sensor still working on it on the slowest sata 3 port, the 3000mb/s one). So, know i have 2 EVO 860 500 gb each, one running OSX High Sierra and the other one running Windows 10.(First installed on another mac (a mac pro 5,1)). The original disk, a 2 tb one, i use for time machine the OSX system. Both disk's perform around 500 mb/s read and right. Windows bench take a little slower, but if i bech both from osx, i see is the same. So, this machine still handle my Motion/3D work and VR hobby creator on an old DK1 headset.
Hope to soon have on hardware upgrade, but going to use this one for a long time yet.