Sampling with multiple ADCS using SPI Interface Raspberry Pi
Would it be necessary to use two separate SPI interfaces to try and
  collect data simultaneously to calculate the current (Voltage across a
  resistor), or would using a single SPI interface with two slaves be
  sufficient?


If you want simultaneous sampling then you can cascade several ADCs so that you have a single SPI interface and a single start convert line. The implication of this is that you read all of the connected ADCs as one serial line.

I have used the AD7988-5 with 16 of them daisy chained like this: -



So, if you still haven't decided on the ADC, maybe consider the above.