Mark and Argon's cautions are very valid. I have a friend who actually quit his job at a certain big box store because of some 'less than ethical' techniques in the TV dept.

Even if they are not doing it on purpose, you would have to consider each TV's source (one hdmi, the other RCA or daisy chained coax), brightness and color settings, so many variables.

With great power comes Awesome irresponsibility.