It is not at all advisable to use a 560 as physx as you will have a 680 which has enough power to give a support in physx without a dedicated physx card.And the only single graphics card to be the fastest in the world singly.
Reminding you(I am sure that you know this already then also I would like to highlight it again) that 500series card are not recommended to be kept as physx card because they are powerful enough to perform with physx capability enabled while playing games and will cause serious problem while using as dedicated physx card with another powerful cards.
The recommended series are
1) 200 series
I would recommend to sell your 560 or keep in backup when your 680 gets worn out.
Selling is the best option if your processor does not require discrete graphics card for displaying on monitor.