yes, IRCTC allots seats on random basis on availablity, but they try to provide seats nearby which are under one PNR. To avail the lower berth quota for the senior citizens, you have to fill the form seperately. In your case, since there are two senior citizens, you fill two forms seperately for them. Ask the booking clerk, to provide seats nearby. on irctc, you yourself can fill the choice of your preferred coach.