Emanuel A. Schegloff: "Overlapping Talk and the 
Organization of Turn-Taking for Conversation," 
Language in Society, 29:1, 1-63, 2000. 

This article provides an empirically grounded
account of what happens when more persons than 
one talk at once in conversation. It undertakes 
to specify when such occurrences are problematic
for the participants, and for the organization of
interaction; what the features of such overlapping
talk are; and what constraints an account of
overlapping talk should meet. It describes the
practices employed by participants to deal with
such simultaneous talk, and how they form an 
organization of practices which is related to the
turn-taking organization previously described by
Sacks et al. 1974. This “overlap resolution 
device” constitutes a previously unexplicated 
component of that turn-taking organization, and
one that provides solutions to underspecified
features of the previous account.