If a person stands on the scale with one foot and weighs 110lbs what does he weigh with both feet on the scale?

His weight doesn't depend on the area of his contact with the scale.

If it did, then we would see differences in our weight when we wear tennis shoes,
fancy shoes, snow boots, socks, and flip-flops.  What really happens is that any
difference we see is only the difference in the weight of whatever we're wearing on
our feet. It has nothing to do with how much of the scale our feet cover.

It doesn't matter whether you stand on the scale with both feet, one foot, or
on tippy toes, on one finger, or on your head. Your weight reads the same.