The problem is that I don't know how to calculate the current flowing into the output when generators are short circuited. The result should be I = V_in/R1, but the current after R1 will split in the current flowing into R2 and the current exiting the voltage divider...
Just to be clear, to find the short circuit output current, you should be shorting the load (what's connected across Vout), not the generator (what's connected across Vin).
If the output is shorted, then you know the voltage across R2.
From that you know exactly how much current is flowing through R2.
If you know the current through R1, and the current through R2, then you (from KCL) know the current flowing through the load.
It may help to actually, on a piece of paper, re-draw the circuit with the source shown explicitly and the short circuit at the output drawn explicitly.