I, too, have the Apricorn Velocity Duo x2 in my 2008 and 2010 Mac Pros. They work very well. Don't hesitate to get one. Samsung EVO or Pro SSDs are fast and available at a good price when they come up on a sale which is just about weekly at Newegg.
Basically, mine did work as a secondary drive but I could not boot OSX from it. Turns out I had an old card. Got a replacement, and that worked fine. Only problem is that one cannot install bootcamp on it and boot from it (at least now Win7, see https://forums.macrumors.com/threads/velocity-solo-x1-and-bootcamp.1681583/). So, you need to keep your Windows system on one of the 4 drive bays. Maybe Win10 does better, haven't been able to try that, yet.