Speaking in Spanish at a meeting of the South American Football Confederation (CSF), Blatter said that if three or four suitable single bids were put forward, the joint ones would be rejected.
"In FIFA, there are decisions which have been taken by the executive committee in view of the organisation of the World Cup," he said.
"These decisions are the following: as soon as there is a candidacy or three or four relevant candidacies that only one country can organise it, we are directly going to reject the double candidacies.
"We have done this at the moment of the decision taken in May 2004 for the 2010 Cup at the moment that Tunisia and Libya put forward a double candidacy."
Spain and Portugal have formally put forward a joint bid to host one of the two tournaments and the Dutch and Belgian football associations, who co-hosted the 2000 European championship, have also submitted a joint bid.
England, Japan, Qatar, Russia and Indonesia have formally declared their interest and the U.S. has said it will also join the race.
The 2010 World Cup will be held in South Africa and the 2014 tournament will take place in Brazil.
Japan and South Korea co-hosted the 2002 World Cup.
