For clarity...
the example of a block experiencing a force of 20 N left and another force of 5N right for a net of 15N does solve the riddle posed.
However, it is not a N3L violation. The 15N is the sum of the forces. Theere is an object experiencing 20N right and another experiencing 20N left. Likewise there is an object experiencing 5N right and another experiencing 5N left.
Lest someone think this violates n3l.
The n3l issues do exists with simultaneity. I just wanted to ensure the above example isnt read by someone as a violation of Newtons third law.