have seen them seperate before but out of the 100+ aquariums that belonged to my coustomers when i worked at the aquarium store more than 2/3's of them noticed that when kept in a group they developed cleaning stations. but like all things it depends on the shrimpi would not say they clean better in pairs..my two work as individuals in the tank, always have done and they were introduced together and came from the same tank when i brought them. Its an instinctive nature to do the job they do and they do not rely on others to carry out their task....
Just an opinion of course, based on personal experience..
Niko
usually the fish are shy from the shrimp lolok well i'll go with one at first and if it is shy and doesnt come out to clean etc then i will add another to see how it goes. Thanks everyone