If you don't kill one they will both save the image in the same directory anyway when they finish. I have noticed sometimes when I open two instances sometimes the second one doesn't display image textures I have applied. Don't know if this is a memory problem or the fact that the first bryce is using the windows routine that does the work and the second can't access it. I'd close the one that is not that far along. This is probably the one you opened second.