O que você quer fazer é chamado CUDA e blender suporta isso .
No entanto da FAQ do Blender CUDA:
Why is Blender unresponsive during rendering?
While a graphics card is rendering, it can not redraw the user
interface, which makes Blender unresponsive. We attempt to avoid this
problem by giving back control over the GPU as often as possible, but
a completely smooth interaction can't be guaranteed, especially on
heavy scenes. This is a limitation of graphics cards for which no
true solution exists, though we might be able to improve this
somewhat in the future.
E a solução é simples:
If possible, it is best to install more than one GPU, using one for
display and the other(s) for rendering.
(embora você provavelmente não goste da "solução") : - (