Hi, you will have to build some hardware to do the job.
OpAmp will help, but first what do you want out of the comparison.
The arduino analog is only positive 0 to +5V.
A standard differential amplifier will when you compare your two LDR outputs.
If LDR1 is Higher than LDR2 you get a positive voltage out.
If LDR2 is Higher than LDR1 you get a NEGATIVE voltage out.
It can be level shifted so that 2.5V is zero difference, above 2.5V is positive and below 2.5V is negative.
The trick is to do it with a single power supply rather than a balance pos/neg supply.
Tom....... 