Via email, William Kelleher directs us to this post and an ongoing debate about whether the “top two” system will increase incumbency advantage and hurt the prospects of third parties and independent candidates. I looked for some literature on the subject and didn’t find much. The problem is that the top two system is rare.
Nevertheless, here is my earlier post on the subject and here is the NY Times roundtable linked therein, including the perspectives of several political scientists. The preliminary consensus was: the effects of the top-two system (e.g., as recently instituted in California) will be smaller than most people think.
If anyone knows of relevant studies, please leave a comment.