I'm not saying it can't be done - but I tried this a couple of times and failed - similarly if a city is due to grow naturally the turn after it starts celebrating it loses half a food box and only grows one
Two caravans will fill the food box, but the celebrations will give you the extra population. On the turn after, if celebrations have ceased, you will receive an extra head of population - assumimg other conditions are correct. If quick growth is needed the only way is to "b" settlers into the city if it is under size 8.
Incidentally, you should never consider growing cities using TWO food caravans - a granary costs only 10 shields more than a caravan - once you have built this you only need one caravan to fill the food box.