could i run the r9 270x with a different nvidia gpu to run physx on?
There used to be a patch that supported an AMD GPU with an NVidia PhysX processor but it hasn't been supported for a few years. And it's known to not be compatible with the latest NVidia drivers and the latest PhysX.
You could never use an AMD card as a dedicated PhysX processor.
mean i's have the amd card as the graphics and the nvidia card as the PhysX card
You can't do it anymore unless the 270x can use three-year old drivers. And even then, it would only work on games that use three-year old PhysX.