Group bookers = 2 4 1 offer

Anyone out there doing group bookings? I believe V9 has the ability to place and Minimum and Maximum on ticket number but for the interim we are wanting to devise a process where we can do a 2 for 1 offer. I know that this would be 50 % off with a minimum and max of 2 tickets purchased - just wondering if anyone is doing it not on Tess but maybe through their web API?