Its all about the drivers, search Nvidia's website and google what drivers work well with other cards. I don't think you can do this with an SLI setup so you'll probably have to find two different drivers that can work on each card separately. Look and see if there is a driver that ONLY does physX, that way it wont interfere with the 560.
i believe you only need one driver package to do all this. nvidia physx can be downloaded separately but normally it was included in the driver package (forceware). for the driver part one driver should be able to handle both 9800 GX2 and GTX560Ti. from the control panel just pick which card you want as a dedicated physx though i do not know if 9800 GX2 can be assigned as a dedicated physx card. (by theories 9800 GX2 as a card
should be able to work as dedicated physx card but since the card itself considered as SLI configuration i can't say for sure. sorry i really can't help on this one :P)