It seems to me that such a system forces the party to unify behind one candidate. I was wondering if you could clarify this point. Very good question about the Geddes and Ribeiro Neto argument on run-offs. What they argue is that run-offs encourage party system fragmentation. The mechanism by which they do this is the following.
Candidates are less constrained by strategic factors in the first round vote.
The result is, generally, more candidates and more fragmentation. Remember, however, Geddes and RN are mainly interested in fragmentation of the legislative party system, because their argument is about how legislative fragmentation makes coalitions harder to build, and the resources generated by corruption more important to presidents. So the last step in their argument has to do with the connection between presidential and legislative elections, which is the following. In systems that use run-off elections for presidents, they always hold the legislative elections on the same day as the first round, when the vote is most fragmented, not the second round, when the presidential race would be narrowed down to two main candidates. Thus, the fragmentation that goes with run-off elections, combined with presidential candidate coattails, spells more fragmentation in the legislative party system than is likely under plurality elections for president.