While I do not have any advice for you I do have some thoughts. The 640 is not one of Adobe's list of supported cards. It is also an entry level card and may not have been considered.
I don't know much about the rest of your system (processor, ram, driver levels, etc, etc, etc) but you may be better off with a newer card.
What are you trying to achieve with the second card?
Just a note ... they're not really concerned for the CC programs with the older list of approved GPU's for the CS series programs. So that list is mostly moot in a CC discussion.
There have been issues with second cards at times and the way the system & the Adobe DVA's are set to use video resources. I'm no expert in this, but as it seems from active watching, most such situations do get resolved by tweaking settings. You just need someone to chime in here with knowledge of that process. As I recall, it oft has included changing something in the PrPro preferences or something whilst one card was disabled, that allowed it to be utilized on booting up with both cards functioning. There are quite a few people working with dual cards.
Moving to the Hardware forum.
wild guess, but what if you go to the nvidia control panel and set cuda to just one of your video cards instead of auto? that setting is under the manage 3d settings.
Thanks for all the replies everyone. Just to answer a few of your questions:
I've been very happy with the performance of the GT 640. It really has worked well with all my projects (mostly h264 footage in 4 0 0 shot on my 60D). The speed with which the Mercury Engine Renders everything in Premiere and Media Encoder has been excellent. However, when I started using a third monitor (plugged into the HDMI port on the 640) I started to notice a SLIGHT drop in performance. I thought the increased CUDA cores being made available from a second card could help ease the load and speed things up again.
My build is a few years old: i7 950 @ 3.07 GHz. 24 GB Ram installed, now running Windows 8.1 off of a 256GB SSD. Capture Drive is a 500 GB SSD. I recently moved this build into a new case and need to upgrade my power supply so I can add the rest of my drives to the build, but most likely I'll be once again striping two 500gb drives into a RAID 0 array for my media cache.
I'm going to play a bit with the settings in the NVIDIA Control Panel and report back. If anyone has any further suggestions please chime in. Thanks again for your help so far everyone.
I'm with Ronin ... I'd disable the second card first, then work out from there to try & get the two cards working ...
Why don't you upgrade to a single newer card. I just saw Best Buy has a GTX 750 Ti that easily supports 4 displays for $170 and is much more current card has 2x Video RAM and more CUDA cores.
That is such an old card that the drivers might not be compatible with Premiere Pro CC 2014