I would like to see the "two party system" break up a bit, and I also agree that the two major parties would not want their own party to lose folks to smaller parties, but I can't see how you can say the existing parties "won't allow" the formation of new political parties. That's just silly. They won't HELP new parties and you aren't going to see the R and D folks entering into public debates with them on a regular basis, but NOTHING stops you from starting your own political party. Nothing on the part of the existing parties, that is.
The only thing stopping you from creating a new political party is yourself.