This is an addendum to luchador's answerluchador's answer.
The transient power dissipation in the two boxes is very different. The following simulation demonstrates this.
simulate this circuit – Schematic created using CircuitLab
Run the simulation for 40 seconds, and plot the expression "I(R1.nA)^2+I(R2.nA)^2", which represents the total instantaneous power in the two resistors.
As I said in my comment, box A will not only heat up more slowly while the pulse is on, it will exhibit a spike in temperature when the pulse ends, because the total instantaneous power dissipated in the resistors is doubled at that moment. Box B will not exhibit such a spike.
(NOTE: If you have trouble running the simulation, see this Meta post.)