...... , there were serious discussions on this at one time because of the different things that both agencies do well. SSI's real good point is their home study materials (from what I am told), for instance. The talks hit the wall, never to return to serious consideration.
The sticking point was SSI's requirement that the dive professionals work through an SSI charter or dive shop. They feel that it allows action to be taken more quickly on "bad apples" in these ranks if there is oversight (quality control, if you will) in the form of the dive shop. Hence, they don't allow completely independent instructors. There is no way that NAUI instructors were going to work with that environment. The other side to it is that SSI dive professionals do turn into representatives for the store without really wanting to. This is not always for the better.
As was said already, NAUI and SSI are different agencies with a different approach to instruction and quality control, neither approach totally better than or worse than the other, but different. I highly doubt that they would merge.