Q: Any hint on which of the 2 graphics cards on a Mac Pro 2013 is giving a graphic problem?

On the 2013 Mac Pro one GPU does all the graphics processing while the other GPU delivers the result to the monitors.

If yo go to System Profiler

OS X: About System Information and System Profiler - Apple Support

yu can see that all monitors are shown connected to only one graphics card

You can run Apple Hardware Test

Using Apple Hardware Test

however, that is not good at detecting graphics problems

I would make an appointment at the Genius Bar of an Apple store.

  Apple Retail Store - Genius Bar

These are custom cards, not "off the shelf". The two cards are different from each other and are not interchangeable.

You will need to deal with Apple to get them fixed. There are no other legitimate parts available, unless you can find a unit that died for other reasons.

I have no idea and I have not seen any article on how the cards work

By default, one GPU is setup for display duties while the other is used exclusively for GPU compute workloads. GPUs are notoriously bad at context switching, which can severely limit compute performance if the GPU also has to deal with the rendering workloads associated with display in a modern OS. NVIDIA sought to address a similar problem with their Maximus technology, combining Quadro and Tesla cards into a single system for display and compute.

from:

http://www.anandtech.com/show/7603/mac-pro-review-late-2013/9

This is from section 9 of a 15-part article reviewing the Mac Pro late 2013.