spectrophotometer design

I want to design a "simple" spectrophotometer in order to obtain the
difference in concentration between two samples. It should have the
following features:

-A light source (laser) that cross a sample, connected to a detector.
-A second laser that cross a second sample, connected to a second
detector.
-Software to obtain the substract of the two signals, in order to
convert in digital or analogical signal.

My idea is that, if the samples contain only water and a same
substance in different concentration, the difference in intensity
between the two detectors will be equivalent to the difference in
concentration.

My question is:

-Can this apparatus work using a detector of intensity, or maybe is
needed separation in wavelengths?
-What kind of detector and light I need, to reach a reasonable
sensitivity?
 

Thank you!