Not sure the problem occurs for everyone. I've don't own a card with this problem but do experience a crash with my 1070 if the render is too fast which suggests a possible synchronisation issue between Poser and the GPU, for example both sides are trying to access the shared memory at the same time (speculation). So in your case possibly adding the second card slows down the GPU side (speculation). The reason for the queue manager suggestion is that version of the render engine doesn't interact with the display so could have a different code path plus eliminates display driver and openGL interaction. The objective here is not to solve the problem but to have some evidence that helps decide if its a Poser or hardware issue. I doubt if this test will yield anything useful so not worth installing queue manager but I thought it worth posting the idea in case someone reading this has the slow render problem and already uses the queue manager.