There is no overriding for JCLLIB statement. Instead you could specify both the Library in order statement. In case if both the library contains the same procedure name , the search order will be left - to- right position in the ORDER clause.
Consider u have the same procedure in both the library,
In case if u want execute the proc from Library1 it should be
// JCLLIB ORDER=(library1,library2)
In case if u want execute the proc from Library2 it should be