After many years using Screenshot captor on windows XP and windows7 without problems.
Unfortunately I am forced to use windows10, and I have the same problem with a laptop connected to a external display.
It seems working fine after configuring sc .exe properties as in attached screenshot.
Before I tried several combinations without luck, such as
Both with text at 100%,
Both at 1680x1050
1680x1050 and 1680x1080 (150%)
1680x1050 and 1680x1080 (100%)
in any configuration the default print screen function, on each screen captures the full screen plus something else (part of the other display or just a blank-white area)
I blame on Microsoft and its beta Windows10 which reminds to much to the old windows millenium.
Windows 10
-bubbajunk