MacPro OS X 10.10.2
2 x MSI GTX 970
Short description of error
Selected dual GTX 970 CUDA rendering only works when doing a normal finale rendering, each GPU working on a tile.
In finale rendering progressive and in 3D viewport rendering there is no speed gain at all.
Actually the 3D viewport rendering is then a tick slower.
As far as I know you should be able to render with two GPUs even in 3D viewport preview.