Using a Dual card for physics is a waste. I think, I might be wrong, that you will only use one core for physics, the other will scratch its belly. To that you have to add that it is a card that consumes its own, I don't know if your source could get into trouble. Really there are few games that use physics.
If you need physics, I would look to sell it and buy a more basic nvidia for physics. I really know that at its time it was very good and expensive, today despite not having DX 11 it can handle many games at good quality, but it is not good to be romantic or nostalgic when talking about hardware, and more with products that depreciate so much. So either you sell it, or back in the drawer again, but nothing of using it for physics. It may seem like harsh advice but it is my point of view, make a profit or keep it as a back up in case your card fails.
The 580 GTX is still a good card but you can also sell both and if you get a good price, buy something more modern, that yields similar or a little more than the 580 GTX but with lower consumption.
A GTX 750 or 750 Ti, would be the ideal cards for physics, for low consumption and great performance in these tasks.
Putting the GTX 295 for physics, is killing flies with cannons, and an extra expense in electricity inappropriate for what is achieved.