Theoretically, yes, each would read 140 pounds, but the person could be leaning to one side or another. (Also, like @filmfann says, two scales on top of each other would read slightly different numbers.)
Also, it might decrease the accuracy/precision of the measurement to use both scales side by side with one foot on each.
AP Statistics time!
Assume that the measurement on each scale is M = W + E, where W is the weight on the scale, and E is a normally-distributed error with a mean of U and a standard deviation of Q.
If you were to use one scale, the measurement will have a mean of (W + U) and a standard deviation of Q. If you were to use two scales and add the measurements together, the sum of the two scales will have a mean of (W + 2U) and a standard deviation of Q*sqrt(2).