Dual Graphics cards in 2009 Mac Pro.

hate to say it, but the only thing to expect with 2 GT 120 video cards is nothing, apart from the extra expense. Mac haven't, don't and won't support dual video cards working together as one, aka CrossFire. (that's what ATI used to call it, I don't know about nVidia). The GT 120 has 512 megs of video ram which is just barely functional in today's market. The 5770/5870 would be a minor step up with 1 gig of video ram. I have a sapphire 7950 with 3 gigs of video ram which does me okay ( though it is a bit outdated). Check out macsales.com. Depending on your budget the RX line would be good too. unfortunately video cards with 3 gigs of video ram or better aren't particularly cheap, and yes, you do have a 2009 Mac Pro so the PCI bandwidth is a bit slower, but do-able. I think 2008-9 is the cutoff point for Mac Pro's, at least in terms of video cards. Earlier than that you won't find much at all . I'd suggest adding an SSD as well, but.... I don't know what your budget is overall, and they do tend to be pricey, as compared to a standard hard drive.....

well, there you go, about the only other thing I could say is that installing a new video card isn't that hard

John B

Those cards can each drive one DVI (or VGA) display plus one Mini DisplayPort display on each card already.

Able to run dual displays ?

Able to run mirror displays ?

Able to drag apps across displays ?

You can already do the things above as soon as you have more than one display. Features are built into MacOS, and every well-behaved Mac Application can do them.

In addition, you can split a window across two displays and it will update "the right way" when you scroll the window.

AMD call their approach CrossFire as mentioned by Grant, Nvidia call theirs SLI. See - NVIDIA SLI Technology|NVIDIA UK

Neither technology has ever been supported on Macs and not all video cards from AMD or Nvidia support it. It is in theory possible to do this in Windows via Boot Camp. Suitable video cards not the GT-120 would likely also require a second power supply.

The Mac Pro 2013 which as standard has two video cards does something similar but this is a proprietary Apple approach using Apple specific video cards. For the Mac Pro 2013 Apple allocate display processing to one card and GPU processing to the other.

There are some software based approaches which can be used on Macs. There is CUDA for Nvidia cards and OpenCL or Metal for both AMD and Nvidia cards. The software has to be written to take advantage of this and again not all cards support these.

Your GT-120 cards are extremely old and extremely low spec and support none of these techniques. So you will not get any additional acceleration.

You can run multiple displays off one card but having two cards does make it possible to add more displays because you have more connections. As standard if you have multiple displays on single or multiple cards the Mac supports 'spanning' whereby all the displays act as a single large display. Yes you will be able to drag apps and windows from one screen to another.

I must confess I have not tried mirroring on a Mac Pro and the normal benefit of mirroring does not apply. The normal use for mirroring is wanting to use the laptop built-in screen at the same time as a projector. It costs nothing to try.

It is possible to get external boxes to allow sending the same single video signal to multiple displays.