Drop in a VRef object and change it's reference to GND or whatever you need.
When I did this, the project would not compile. I got a vdac not connected error
I believe the sequencing SAR ADC uses the hardware multiplexer that only allows input connections to pins so you will have to connect your two single ended inputs as differential with the second pin going to ground for each pair.
You can use GrC recommendation but you would have to use a SAR + a mux where you control the mux in your code.
The SEQ SAR as BrRu indicated provides compile/build errors when you connect the inputs to anything but a analog pin.