hi REB,
Looking at your weight ranges, 60 thru 150 lbs, with a difference weight
of 20lbs, I make that about 70 states, ie; 150-130, 149-129 etc...
My first suggestion would a load cell under each platform, depending upon the outside dimensions of your platform.
You would need some amplification of the load cell signal, before you fed it
into a analog to digitial convertors. Depending upon the resolution you required you may be able to use the ADC in the PIC.
You could use a 'voltage threshold window' method by comparing the outputs of the signal amplifiers, no PIC required.
Just amplify the load cell signals, sum/difference the two signals into a comparator and use the output to sound an alarm.
This would be my preferred solution, looking at the resolution you state.
You can either buy the finished load cells [ there not cheap ] or attempt to use strain gauge elements
[ I would not recommend this route, unless you have lots of experience]
Regards
EricG