Vsomething
  *--------------------- ADC1
  |
  Rfix
  |
  *--------------------- ADC0
  |
  Rvar
  |
gnd

with this setup adc1 will read x and adc0 wull read y

so <b>whatever Vsomething is</b> (cheap reference) 

x     (Rfix + Rvar)
--- = -------------
y     Rvar