I would hope that any proportional voting system that was introduced would also break the two-party duopoly, but I suppose that's not a guaranteed consequence.

For example, if IRV was more widely adopted, you might still find that only the two big parties ever win seats (although by removing the spoiler effect, third parties become a viable threat which could act to steer the big parties towards more representative policies rather than them relying on the "lesser of two evils" effect).

Conversely, it could be possible to have somewhat popular third parties without a proportional voting system, as is seen in Canada or the UK, but those countries still see the biggest party at the national level always being one of the two main parties.