To the OP: How many radiant elements are in your toaster? One for each
side of each slice or just two (unlikely) or three? If you have less
than four no wonder you're getting a different result with one slice.

If you take two (or really any number) of slices and toast them in a
domestic toaster, depending on the darkness control, you'll get a
different result than if you toast them in a commercial toaster. The
difference is the power of the radiants. The radiants effectively burn
the outside layer of the bread; in the limp-wristed powerless domestic
version they take so long to do so that the heat soaks through the
bread and drives out the moisture from the entire slice thus making a
crisper more brittle toast. In the commercial version the radiants
burn the outside quickly bringing it up to the desired color before
the moisture can be removed from the inside. Try it next time you eat
in a luncheonette or diner.

What is necessary is for someone to produce a high quality toaster
that actually does what is pretends to do. You need a scanner that
measures by pixel (or crumb) on each side of the bread, determining
the average pixel darkness and ejecting the slice when it reaches a
preset color. It would also have to test if any one or group of pixels
exceeded a "burnt" threshold to avoid being thrown off by irregular
bread surfaces. The moisture content should also be factored in (user
settable) by measuring the humidity level of the output air. Some
electronics nerd needs to get on the stick right now!