Actually, it's going to make them a lot more likely, initially : it won't be 10 demigods, it will be 8 old and 2 new. For a coupla weeks after 1.2 is released, everybody's gonna want to fiddle with the two new demigods...
This is actually a really good point. If they release these a new DG, pull a lot of people back, and then only one player per team can pick that guy, people are going to be annoyed because you'll practically have to host your own game to try something new.
If both are released together (which might or might not happen) it wouldn't be so bad, but I think that the fact that they're releasing two new DGs is only an argument in favor of not bothering to ban doubles. Early on lots of people want to try the new hero, later on there's a greater variety and less likelihood of doubles.