this makes sense because as you found out already: vvvv/stride is running from the primary card. and: only from that card! ie the second card is not adding any power, it only costs additional power to copy data, rendered by the main card over to the second card.
yes, because vvvv/stride is not using OpenGL, but DirectX11.
obviously this is really odd. what behavior do you get if you arrange all 5 windows only on desktops that are running on the main card, ie non-fullscreen. just to see if the main card would handle 5 windows properly.
as noted above, this may not even help, as still only the main card would do the rendering.