Supercapacitor Emergency Power Supply UPS
you want 12v x 2A = 24Watts of output power for 15 seconds
if we assume efficiency of 90% and an input voltage of 5v, the average input current would be 
24/0.9/5 = 5.33A
this would be the average current, the peak current would be much worse!(typically 2 to 3 times more)
internal mosfet resistance is 0.17ohm according to datasheet, so the dissipation would be too much for this tiny package even for 15seconds.
you should use another IC which supports external mosfet or another part with much lower internal mosfet resistance.