Currently, you can only blitz 1 players - keeping in mind that that will be 1 player that is not already at the line (based on formation set settings). I still don't think 3-4 is quite right as there should be 4 players normally at the line, but that's another topic. Anyway, the blitz setting on the formation set is a blitz distribution settings, sort of like the rush/pass distributions on the offensive side. They do not have to add up to 100, but it's probably easier to understand it as a percentage. For instance, if you had only 2 players set and one was 20 and the other was 10, the first player would blitz 67% of the time and the second 33% of the time, so it might be easier to set them to 67 and 33.
The idea of a 2-man blitz is possible but that would probably be a feature we add at a later date.