what accuracy do you need ? you make two contradictory statements !
I want one robot to find the distance(need not be accurate) between the other robot
AND
method that I thought was use relative positioning , i.e start all the robots from the same point and as the robot moves increment/decrement a (x,y) variable stored in it based on the direction it moves . However this method is very basic,inaccurate and non-scalable
you could use optical encoders which might not give huge errors but depends on different factors
NOTE :- accuracy and errors are relative to you so please specify
( at the begining even i didnt know this 
but we should know this )